Display Hardware Compositing in Weston

The DRM compositor in Weston calls drmModeAddFB2WithModifiers() to associate a DRM framebuffer ID with the dma-buf it receives via Linux DMA-BUF Unstable V1 Protocol. It then uses the framebuffer ID to present the dma-buf to one of the available display hardware planes using drmModeSetPlane(), in the legacy path, or drmModeAtomicCommit(), in the atomic modeset path. This takes the form of a flip, where the reference to the new buffer is swapped in during vblank.