gl: Use a uniform to flip R and B colors on GLES

This allows us to decide when the R and B color channels should be
flipped with a much better granularity.

For instance, when using GLX_EXT_texture_from_pixmap to create a GL
texture from a surface we don't need to swap the R and B channels, as
the internal representation of the texture data will already have the
appropriate colors.

We also don't need to flip color channels when blitting from a texture.

gdk/resources/glsl/gles2-texture.fs.glsl

@@ -1,12 +1,16 @@
 precision mediump float;
 
 uniform sampler2D map;
+uniform int flipColors;
 
 varying highp vec2 vUv;
 
 void main() {
   vec4 color = texture2D(map, vUv);
 
-  /* Flip R and B around to match the Cairo convention */
-  gl_FragColor = vec4(color.z, color.y, color.x, color.w);
+  /* Flip R and B around to match the Cairo convention, if required */
+  if (flipColors == 1)
+    gl_FragColor = vec4(color.z, color.y, color.x, color.w);
+  else
+    gl_FragColor = color;
 }