Allow NULL in glClear*Buffer and glClear*Tex functions
Allow NULL in glClear*Buffer and glClear*Tex functions

* The spec says that if data is NULL, it acts like a buffer filled with
  zeroes, so we can just memset.

victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc