GUI scale (#737):
- add display.gui-scale setting (0 - auto, 1..4 - fixed integer scale)
- auto mode picks the largest scale keeping at least 640x480 UI units
- apply scale in GUI (root size, camera, cursor, tooltips, scissors)
- HUD layout and ModelViewer use scaled coordinates
- add GUI Scale trackbar to display settings
Volume contrast (#399):
- add audio.volume-contrast setting (100% - unchanged sounds,
0% - loud sounds are lowered to the level of quiet ones)
- loudness (RMS) of a sound is calculated once and cached
- add trackbar to audio settings and reset page
