Use-case for CTRC background color?

[Harish Krupo]

Hi,

This patch [1] introduces the background crtc color property. This
property is available on some display controlers and allows setting a
non-black colors for pixels not covered by any plane (or pixels covered
by the transparent regions of higher planes). I would like to implement
a userspace consumer of this propery in weston.
The primary use case for this property is for compositors to set the
background color. I am planning on implementing it as follows:
* The desktop client would bind to the weston-desktop-shell protocol.
* The weston-desktop-shell would have a wrapper for the wl_output
object.
* The client would create a wrapper for wl_output (maybe called
weston_desktop_shell_output).
* This output interface will have an event to advertise its capability
to set the background color directly.
* Once the capability is found and if the client wishes to set the
background color, it would use the set_background_color request in the
weston_desktop_shell_ouput interface to set the color by providing the
r,g,b primaries.

I have not completely thought this through, so I am sure this is
incomplete and will require modification. I would like to know your
comments and suggestions on the above method (or even if we could use
this property differently) before I begin the implementation.

Thank you
Regards
Harish Krupo

[1] https://patchwork.freedesktop.org/series/50834/