cmake: Minor style fixes
Use alpha to coverage to AA color-keyed textures

This currently only works if alpha blending isn't enabled.
When we are already alpha blending we should just include the src alpha
from the color-keyed texture in the blend factor.

Performance impact should be minor, but we may still want to add a
separate (from base AA) config option for alpha to coverage.

Also, we currently (almost) always enable the ColorKey render state,
even if no color keyed texture is being used. This is a bit wasteful.
Exit mouse look mode on focus loss

Otherwise we may end up warping the mouse cursor while we don't 'own'
it, which can be very annoying.
Fix comparison of signed and unsigned integers
Fix menu cursor trail breaking after loading game

Starting a game and then exiting to the menu pauses time...

Was broken in commit 3faac5.
Reset the menu cursor trail when opening the menu

Fixes a sudden jump in trail positions.
Set TEXTURE_MAX_LEVEL for non-mipmapped textures
Add more checks to DEBUG_EXTRA