texture: Refactor downloading

texture: Refactor downloading

Pass a format do GdkTextureClass::download(). That way we can download
data in any format.

Also replace gdk_texture_download_texture() with
gdk_memory_texture_from_texture() which again takes a format.

The old functionality is still there for code that wants it: Just pass
gdk_texture_get_format (texture) as the format argument.