This is a completely new screenshooting protocol designed to support:
- color management testing by adding the "blending" source
- KMS testing by adding the "writeback" source
- output decorations testing by adding the "full_framebuffer" source
- proper buffer size negotiation instead of guessing from wl_output
- compositor chosen pixel format, primarily for "blending" source
- proper indication of screenshot failure
- dmabuf target buffers, linear only

This new protocol should be good enough to publish as a Weston public
extension. Hence install it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>