Like physical size, subpixel arrangement, etc, transform advises of a
physical transform of a head, if present.
This commit adds the transform member and setter to weston_head, however
it is currently unused.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
[daniels: Extracted from one of Lucas's patches.]
Signed-off-by: Daniel Stone <daniels@collabora.com>