texture: Add gdk_texture_diff()

texture: Add gdk_texture_diff()

... and use it in rendernodes.

Setting up textures for diffing is done via gdk_texture_set_diff() which
should only be used during texture construction.

Note that the pointers to next/previous are allowed to dangle if one of
the textures is finalized, but that's fine because we always check both
textures' links to each other before we consider the pointer valid.