texture: Make format a property of GdkTexture

texture: Make format a property of GdkTexture

For MemoryTexture, this is a simple change.

For GLTexture, we need to query the format at texture creation. This
sounds like a bad idea and extra work until one realizes that we'd
need to do that anyway when using the texure the first time - either
when downloading, or when trying to use it in a rendernode, where we
will soon need that information to determine if the texture prefers high
depth.