link png library in code on osx

Fix frozen hog rendering

Fix check for flag

hopefully closes issues 541, 312 and 503

yeah. this was a bad idea. the minor difference in thaw speed vs last hog of team never unthawing.

prevent an old abuse of parachute to land safely

fix fade to black in training

PhysFS-aware check for directory existence. Should fix issue #511.

skip frozen hogs in switch

Add missing feedback button translation

This has bothered me for the longest time. Move texture so it doesn't overlap. I'd initially had a test for isPaused, but jumping texture bothered me.

our cmake modules have become good enough, prefer them over system ones