Merged gamma files into monitor files.

src/CMakeLists.txt

src/cocoa_gamma.c

src/cocoa_monitor.m

src/gamma.c

src/monitor.c

src/win32_gamma.c

src/win32_monitor.c

src/wl_gamma.c

src/wl_monitor.c

src/x11_gamma.c

src/x11_monitor.c