drm backend: set "Broadcast RGB" = "Full" for all connections
I have a new monitor which has three similar modes:
- 3840x2160 @ 59.939999 Hz
- 3840x2160 @ 60.000000 Hz
- 3840x2160 @ 59.997002 Hz
The last one works fine, but the first two have issues displaying colors, because the "Broadcast RGB" property on the connector defaults to "Auto" which means "Limited 16:235", squashing the dynamic range.
Since there is really no way for a sway user to know that 3840x2160@60Hz displays colors poorly, while 3840x2160@59.997Hz will be fine, I was using the wrong mode for weeks.
This patch causes wlroots to always set "Broadcast RGB" = "Full", which I think most people want.