Create GL context with DEBUG_BIT set when SOKOL_DEBUG is defined.

[castano]

This mirrors the behavior of D3D11 and allows applications to use OpenGL debug output functionality.

Note that in non-debug contexts, the OpenGL implementation may not generate messages even if debug output is enabled, so this is required for debug output to work in all cases.

Questions:

• Should we check for the presence of KHR_debug before enabling this?
• Should this be an app_desc parameter instead of a compile time definition?

[castano]

Any updates about this? It'd be great to merge this to make it easier to maintain my sokol_app branch.

[floooh]

Thanks for the reminder. I'll try to give the PR a whirl this evening or over the weekend.

[floooh]

Ok, I'm not seeing any problems on my old and cheap Intel GPU laptop, guess that's a good sign ;)

Merging...

[castano]

Thanks!