texture: Add GdkMemoryTexture

texture: Add GdkMemoryTexture

GdkMemoryTexture is a texture implementation for holding data in memory
(read: GBytes). You specify the GdkMemoryFormat that data is in and off
you go.

Renderers can use this to add uploads in various different formats and
don't need to fallback to GDK doing the conersion on the CPU.

Supported formats can be extended if we need new ones, for now I just
added the relevant ones for Cairo and GdkPixbuf.

The constructor is also private still, because I'm not sure we want to
export GdkMemoryFormat.
Wrappers that do from_cairo_surface() and for_pixbuf() do exist though.