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.