Implement wlr_export_dmabuf_unstable_v1 protocol

This PR adds support for the wlr_export_dmabuf_unstable_v1 protocol.

• Add dma-buf support to backends
  • On DRM via GBM
  • On other backends via the GL extension (see #906) not possible, that would require to render to a texture
• Support capturing clients
• Add byte buffer support to DRM
• Implement the protocol
  • Basic functionality
  • Frame synchronization
  • Timestamps
  • Frame locking
• Add example client (cc @atomnuker)

Fixes #709 (closed)