texture: Add gdk_texture_download_texture()

texture: Add gdk_texture_download_texture()

A private vfunc that downloads a texture as a GdkMemoryTexture in
whatever format the texture deems best.

There are multiple reasons for this:

 * GLES cannot download the Cairo format. But it can download some
   format and then just delegate to the GdkMemoryTexture implementation.

 * All the other download vfuncs (including the ones still coming) can
   be implemented via download_texture() and delegation, making the
   interface easier.

 * We want to implement image loading and saving support. By using
   download_texture(), we can save in the actual format of the texture.

 * A potential GdkCompressedTexture could be implemented by just
   providing this one vfunc as a compress() step.