Remove all the roadblocks we've put up to keep implicit modifiers
out. Our importing code already handles them as a signal that says
'No modifiers, please!'. Now we just hope for the best and pass
things along.
This is necessary since some drivers won't produce any explicit
modifiers.
Add a builder for a new GdkTexture subclass that
wraps dmabuf buffers on Linux. For now, this is
just an API. The implementation will follow in
subsequent commits.
