Add environment flags for forcing a certain OpenGL backend into use

As we have found out, many embedded devices do not work optimally at the highest OpenGL version that is supported by it and detected by our findIdealxx functions.

We need to be able to try quickly which backend works best for different devices. An environment variable should be added, so these backends can be tried without recompilation.

These are what I have run into this far:

• i.MX8 works much faster with GL3 backend instead of the detected GL4 -> QUICK3D_FORCE_OPENGL_BACKEND="GL3"
• R-Car D3 fails to render many things with ES3.1 backend -> QUICK3D_FORCE_OPENGL_BACKEND="ES3.0"
• R-Car D3 render some scened much faster with ES2 backend -> QUICK3D_FORCE_OPENGL_BACKEND="ES2"

At least these need to be added. When implemented this way, it's easy to add more options if we run into them (or just go ahead and add them at the same time).