Add fps limiter for background window.
Background fps limit should not be 0 (possible div by 0).
Introducing constantAnimation variable

plus quick fix tor displacers
Merge pull request #863 from sfalexrog/fixes

Fix overly accurate throwing
Merge pull request #864 from AndO3131/drawingRoutines

Introducing constantAnimation variable
Units holding weapons are handled better
Else condition may be needed sometimes
don't load layouts with bad slot allocations
Merge pull request #865 from AndO3131/drawingRoutines

Variable _canHoldWeapon used in Armor class
fix projectile drawing based on direction
better names for vars

because upper may be lower than lower
Fix ethereal walking sound
Fix No Psionics mod not affecting aliens
fix unit sprite location while meleeing
initialize Screen::_flags (avoid UMR)
Merge pull request #866 from ajschult/umrBranch

initialize Screen::_flags (avoid UMR)
make all the air above the landing craft visible.
initialize BattlescapeState::_reserve to avoid UMR
only look at event button for button events
Merge pull request #867 from ajschult/umrBranch

initialize BattlescapeState::_reserve to avoid UMR