Emulate full-screen brightness/gamma ajustment
At least some wine games change the brightness by adjusting the gamma value of the display by using xrandr. For example when running Tomb Raider Underworld in a virtual wine desktop (see also #63 (closed)) - here changing the in-game brightness not only changes the brightness within the virtual desktop but of the whole screen. It is probably a bug in wine that should do this only when a client runs in fullscreen mode, and not in a virtual desktop, but in any cases the result when running this game via wine in virgl is that the game is too dark, because that gamma change is not applied. It appears that the gamma value used is actually 1.4, which corresponds to the lowest intensity when run on the host.
I'm not sure whether we really want to emulate the gamma correction this, and whether it should be done with virgl, or it would actually be a qemu thing though.