Add GDK_GL env var and GdkGLFlags

This moves the GDK_ALWAYS_USE_GL env var to GDK_GL=always.
It also changes GDK_DEBUG=nogl to GDK_GL=disable, as GDK_DEBUG
is really only about debug loggin.

It also adds some completely new flags:

 software-draw-gl:
   Always use software fallback for drawing gl content to a cairo_t.
   This disables the fastpaths that exist for drawing directly to
   a window and instead reads back the pixels into a cairo image
   surface.

 software-draw-surface:
   Always use software fallback for drawing cairo surfaces onto a
   gl-using window.  This disables e.g. texture-from-pixmap on X11.

 software-draw:
   Enables both the above.

gdk/gdk.c

@@ -134,6 +134,14 @@ static GMutex gdk_threads_mutex;
 static GCallback gdk_threads_lock = NULL;
 static GCallback gdk_threads_unlock = NULL;
 
+static const GDebugKey gdk_gl_keys[] = {
+  {"disable",               GDK_GL_FLAGS_DISABLE},
+  {"always",                GDK_GL_FLAGS_ALWAYS},
+  {"software-draw",         GDK_GL_FLAGS_SOFTWARE_DRAW_GL | GDK_GL_FLAGS_SOFTWARE_DRAW_SURFACE},
+  {"software-draw-gl",      GDK_GL_FLAGS_SOFTWARE_DRAW_GL},
+  {"software-draw-surface", GDK_GL_FLAGS_SOFTWARE_DRAW_SURFACE},
+};
+
 #ifdef G_ENABLE_DEBUG
 static const GDebugKey gdk_debug_keys[] = {
   {"events",        GDK_DEBUG_EVENTS},
@@ -149,7 +157,6 @@ static const GDebugKey gdk_debug_keys[] = {
   {"eventloop",     GDK_DEBUG_EVENTLOOP},
   {"frames",        GDK_DEBUG_FRAMES},
   {"settings",      GDK_DEBUG_SETTINGS},
-  {"nogl",          GDK_DEBUG_NOGL},
   {"opengl",        GDK_DEBUG_OPENGL},
 };
 
@@ -248,6 +255,7 @@ void
 gdk_pre_parse_libgtk_only (void)
 {
   const char *rendering_mode;
+  const gchar *gl_string;
 
   gdk_initialized = TRUE;
 
@@ -268,6 +276,12 @@ gdk_pre_parse_libgtk_only (void)
   }
 #endif  /* G_ENABLE_DEBUG */
 
+  gl_string = getenv("GDK_GL");
+  if (gl_string != NULL)
+    _gdk_gl_flags = g_parse_debug_string (gl_string,
+                                          (GDebugKey *) gdk_gl_keys,
+                                          G_N_ELEMENTS (gdk_gl_keys));
+
   if (getenv ("GDK_NATIVE_WINDOWS"))
     {
       g_warning ("The GDK_NATIVE_WINDOWS environment variable is not supported in GTK3.\n"

gdk/gdkgl.c

@@ -335,7 +335,8 @@ gdk_cairo_draw_from_gl (cairo_t              *cr,
 
   /* For direct paint of non-alpha renderbuffer, we can
      just do a bitblit */
-  if (source_type == GL_RENDERBUFFER &&
+  if ((_gdk_gl_flags & GDK_GL_FLAGS_SOFTWARE_DRAW_GL) == 0 &&
+      source_type == GL_RENDERBUFFER &&
       alpha_size == 0 &&
       direct_window != NULL &&
       direct_window->current_paint.use_gl &&
@@ -415,7 +416,8 @@ gdk_cairo_draw_from_gl (cairo_t              *cr,
 
     }
   /* For direct paint of alpha or non-alpha textures we can use texturing */
-  else if (source_type == GL_TEXTURE &&
+  else if ((_gdk_gl_flags & GDK_GL_FLAGS_SOFTWARE_DRAW_GL) == 0 &&
+           source_type == GL_TEXTURE &&
            direct_window != NULL &&
            direct_window->current_paint.use_gl &&
            trivial_transform &&
@@ -602,7 +604,8 @@ gdk_gl_texture_from_surface (cairo_surface_t *surface,
   guint target;
 
   paint_context = gdk_gl_context_get_current ();
-  if (paint_context &&
+  if ((_gdk_gl_flags & GDK_GL_FLAGS_SOFTWARE_DRAW_SURFACE) == 0 &&
+      paint_context &&
       GDK_GL_CONTEXT_GET_CLASS (paint_context)->texture_from_surface &&
       GDK_GL_CONTEXT_GET_CLASS (paint_context)->texture_from_surface (paint_context, surface, region))
     return;

gdk/gdkglobals.c

@@ -34,4 +34,5 @@ GList              *_gdk_default_filters = NULL;
 gchar              *_gdk_display_name = NULL;
 gchar              *_gdk_display_arg_name = NULL;
 gboolean            _gdk_disable_multidevice = FALSE;
+guint               _gdk_gl_flags = 0;
 GdkRenderingMode    _gdk_rendering_mode = GDK_RENDERING_MODE_SIMILAR;

gdk/gdkinternals.h

@@ -85,10 +85,17 @@ typedef enum {
   GDK_DEBUG_EVENTLOOP     = 1 << 10,
   GDK_DEBUG_FRAMES        = 1 << 11,
   GDK_DEBUG_SETTINGS      = 1 << 12,
-  GDK_DEBUG_NOGL          = 1 << 13,
+  GDK_DEBUG_OPENGL        = 1 << 13,
-  GDK_DEBUG_OPENGL        = 1 << 14
 } GdkDebugFlag;
 
+typedef enum {
+  GDK_GL_FLAGS_DISABLE                = 1 << 0,
+  GDK_GL_FLAGS_ALWAYS                 = 1 << 1,
+  GDK_GL_FLAGS_SOFTWARE_DRAW_GL       = 1 << 2,
+  GDK_GL_FLAGS_SOFTWARE_DRAW_SURFACE  = 1 << 3,
+} GdkGLFlags;
+
+
 typedef enum {
   GDK_RENDERING_MODE_SIMILAR = 0,
   GDK_RENDERING_MODE_IMAGE,
@@ -99,6 +106,7 @@ extern GList            *_gdk_default_filters;
 extern GdkWindow        *_gdk_parent_root;
 
 extern guint _gdk_debug_flags;
+extern guint _gdk_gl_flags;
 extern GdkRenderingMode    _gdk_rendering_mode;
 
 #ifdef G_ENABLE_DEBUG

gdk/gdkwindow.c

@@ -287,16 +287,11 @@ create_surface_accumulator (GSignalInvocationHint *ihint,
 
 static GQuark quark_pointer_window = 0;
 
-static gboolean always_use_gl = FALSE;
-
 static void
 gdk_window_class_init (GdkWindowClass *klass)
 {
   GObjectClass *object_class = G_OBJECT_CLASS (klass);
 
-  if (g_getenv ("GDK_ALWAYS_USE_GL"))
-    always_use_gl = TRUE;
-
   parent_class = g_type_class_peek_parent (klass);
 
   object_class->finalize = gdk_window_finalize;
@@ -1424,7 +1419,7 @@ gdk_window_new (GdkWindow     *parent,
                     G_CALLBACK (device_removed_cb), window);
 
 
-  if (always_use_gl)
+  if ((_gdk_gl_flags & (GDK_GL_FLAGS_ALWAYS | GDK_GL_FLAGS_DISABLE)) == GDK_GL_FLAGS_ALWAYS)
     {
       GError *error = NULL;
 
@@ -2730,7 +2725,7 @@ gdk_window_ref_impl_surface (GdkWindow *window)
 GdkGLContext *
 gdk_window_get_paint_gl_context (GdkWindow *window, GError **error)
 {
-  if (_gdk_debug_flags & GDK_DEBUG_NOGL)
+  if (_gdk_gl_flags & GDK_GL_FLAGS_DISABLE)
     {
       g_set_error_literal (error, GDK_GL_ERROR,
                            GDK_GL_ERROR_NOT_AVAILABLE,