Blender Git Loki

OpenGL: enhance debug output

When running blender --debug-gpu

Display which debug facilities are available. One of these, in order of preference:
- OpenGL 4.3
- KHR_debug
- ARB_debug_output
- AMD_debug_output

All messages are logged now, not just errors. Will probably turn some of these off later.

GL_DEBUG_OUTPUT_SYNCHRONOUS lets us break on errors and backtrace to the exact trouble spot.

Callers of GPU_string_marker no longer pass in a message length, just the message itself (null terminated).

Apple provides no GL debug logging features.