1. it apparently is ineffective
2. if it was effective, it's current behavior would be not exactly helpful
   (sets __GL_YIELD to NOTHING, causing busy waits on doublebuffer swapping)
3. it does for sure pollute the doublebuffer/usleep detection (setenv is set to override),
   ie. the overehad detection code gets a different opinion on __GL_YIELD than libGL

REVIEW: 111858
CCBUG: 322060