[Demo] FPL_OpenGL automatically fallback to legacy when modern OpenGL is not available

[f1nalspace]

The current FPL_OpenGL demo requires to set the MODERN_OPENGL define.
On most systems this works, because dedicated GPUs supports at least GL 3.3 - but on virtual machines, you get at most OpenGL 2.1 :-(

Remove the define and use modern OpenGL first, if this fails then fall back to the legacy one.