* fastest compiler optimization for xBRZ scaler

xbrz.cpp is always compiled with -O3
+ console command to purge cached textures

cleartexturecache CCMD will not purge used textures like fonts
+ developer profiling of texture precaching
+ multi-threaded xBRZ scaler

uses Grand Central Dispatch (libdispatch) to upscale image slices in parallel
new CVARs:
gl_texture_hqresize_multithread: on/off
gl_texture_hqresize_mt_width: texture width threshold, 2..1024
gl_texture_hqresize_mt_height: texture height threshold, 2..1024