Add cmake configuration for using OpenGL on the Raspberry Pi 4.

[vanfanel]

Previously, for the Pi4 optimized builds, only GLES2 cmake configs were present. This adds an OpenGL cmake configuration for the PI4, so full OpenGL (legacy GLX or up-to-date GLVND) is used.

[vanfanel]

@djyt I also added a fix for the flickering backgound on GLES on certain configurations.

[vanfanel]

@djyt I modified the flashing backgound correction (context must be cleared before each frame is rendered to prevent this). Also, corrected the spacing: I was using a mix of 4-spaces and tabs, which looks ugly now, so I am using 4-spaces always, as the rest of the engine code seems to do.