One approach to dmabufs in glarea

This branch keeps the current code for creating gl textures, and just exports them as a dmabuf texture in the end.

This seems to work surprisingly well, but for one thing ... the texture ends up upside-down when it is offloaded (I added an offload to the shadertoy demo). The branch contains fixes to the offload code to handle this correctly by setting buffer transforms when attaching dmabufs to subsurfaces.