Start crash fixes

Two little crashes crept into the game while I wasn't looking. a) do not use _lzcnt_u32() anymore for texture size calculations as it silently yields wrong results on older CPUs. b) do not use _popcnt() anymore as it produces an INVALID_INSTRUCTION crash on older CPUs.

Huge thanks to qDan, SkEyE, meetnik for their patience and their aid in getting to the bottom of this.