When rendering with headless builds, show an error instead of crashing.
Previously GPU_backend_init was called indirectly from
DRW_opengl_context_create, a new function is now called from the window
manager (GPU_backend_init_once), so it's possible to check if the GPU
has a back-end.
This also disables the `bgl` Python module when building WITH_HEADLESS.
Reviewed By: fclem
Ref D15463
00dc747702