gltexture: Optionally take a sync object

gltexture: Optionally take a sync object

Add a new function to TextureBuilder that takes a GLsync that
requires internal code to wait on before using the texture.

Somewhat sneakily, we don't take the sync if syncs are not supported by
the current GL context.
As public API has no code to query the sync for the destroy notify, this
is fine and it means we don't have to do the check every time we want to
call gdk_texture_get_sync() internally.