doc: QtGL/QtPlayer: improve code samples portability

NOTE: this is a draft because it needs @robUx4 approval to fixup the fixup: doc: QtGL: use QMutexLocker instead of std::lock_guard commit into the doc: QtGl: replace std::mutex with QMutex commit, as suggested by @chub a while ago.

This MR re-includes the commits that were sent a while ago by @robUx4 on the mailing list.

They mainly clean up the code for plugins, use Qt mutex objects instead of C++ std ones since they are not supported with the thread mode we use in MinGW, clean up OpenGL function usage and improve compatibility with OpenGL ES2.