Add GdkRGBA struct to supersede GdkColor

GdkRGBA is a boxed struct similar to GdkColor, with the difference
that it stores alpha information as well, and colors are stored in
[0..1] doubles, in the cairo spirit.

gdk_cairo_set_source_rgba() has been also added to allow easier handling
of this new type.

docs/reference/gdk/gdk-docs.sgml

@@ -24,6 +24,7 @@
     <xi:include href="xml/regions.xml" />
     <xi:include href="xml/drawing.xml" />
     <xi:include href="xml/colors.xml" />
+    <xi:include href="xml/rgba_colors.xml" />
     <xi:include href="xml/visuals.xml" />
     <xi:include href="xml/cursors.xml" />
     <xi:include href="xml/windows.xml" />

docs/reference/gdk/gdk3-sections.txt

@@ -259,6 +259,21 @@ GDK_TYPE_COLOR
 
 </SECTION>
 
+<SECTION>
+<TITLE>RGBA Colors</TITLE>
+<FILE>rgba_colors</FILE>
+GdkRGBA
+gdk_rgba_copy
+gdk_rgba_free
+gdk_rgba_parse
+gdk_rgba_equal
+gdk_rgba_hash
+gdk_rgba_to_string
+
+<SUBSECTION Standard>
+GDK_TYPE_RGBA
+</SECTION>
+
 <SECTION>
 <TITLE>Drawing Primitives</TITLE>
 <FILE>drawing</FILE>
@@ -595,6 +610,7 @@ gdk_window_create_similar_surface
 gdk_cairo_create
 gdk_cairo_get_clip_rectangle
 gdk_cairo_set_source_color
+gdk_cairo_set_source_rgba
 gdk_cairo_set_source_pixbuf
 gdk_cairo_set_source_window
 gdk_cairo_rectangle