optimized code + scale2x and HQX

Scale2x
HQX
much faster versions of _zoomSurfaceY() for the common cases of x2 and x4 scale factors (640x400 and 1280x800)
inlining SavedBattleGame::getTile(), it's the most-called method in the whole project
an option to use SDL_ASYNCBLIT which seems to improve performance significantly with multiple cpu cores
plus a couple of other small things
merge conflict resolved