Add OpenGL debug logging using QOpenGLDebugLogger

Allow debug message logging from the OpenGL driver if it supports the
extension GL_KHR_debug.

This adds two options to the config file kritadisplayrc:

• EnableOpenGLDebug: Set to true to enable logging. Default is false.
• OpenGLDebugSynchronous: Set to true to log in synchronous mode. This is useful for setting a breakpoint at openglOnMessageLogged and checking the backtrace to see the cause of the message. Default is false for asynchronous mode.

Alternatively, the environment variable KRITA_OPENGL_DEBUG can also be
used to enable the logging:

• (not set): Use the settings from the config file.
• sync: Enable logging in synchronous mode.
• (other values): Enable logging in asynchronous mode.

Differential Revision: https://phabricator.kde.org/D7132