gltexture: Rewrite downloading code

gltexture: Rewrite downloading code

For non-gles, make it handle unpremultiplied formats,
and everything else, by downloading the texture in its
preferred format and, in most cases, doing a
gdk_memory_convert afterwards.

For gles, keep using glReadPixels, but handle cases
where the gl read format doesn't match the texture
format by doing the necessary swizzling before calling
gdk_memory_convert.