A problem with textures is that they can become too big for GPU memory,
which would require tiling. But for tiling we only need to download
the pixels needed by the tile.
Similarly, there might be interest to not upload full textures if a
renderer knows it only needs a small part.
Both of these methods require the ability to specify an area of the
texture to be downloaded. So change the download vfunc to include
this parameter now before we add even more textures later.
A private gdk_texture_download_area() function has also been added, but
nobody is using it yet.
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.
