Do not change the gamma of the display on startup.

Added in rP258 to (most likely) have some control over brightness, this conflicted with dimming utilities and would not play nice with starting 0 A.D. in windowed mode.

If gamma handling were to be reintroduced later, it should only affect the 0 A.D. window.

Reviewed By: vladislavbelov

Differential Revision: https://code.wildfiregames.com/D1976