Fix T99706: Crash rendering with headless builds

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

source/blender/python/intern/bpy_interface.c

@@ -259,7 +259,9 @@ static struct _inittab bpy_internal_modules[] = {
     {"mathutils.kdtree", PyInit_mathutils_kdtree},
 #endif
     {"_bpy_path", BPyInit__bpy_path},
+#ifdef WITH_OPENGL
     {"bgl", BPyInit_bgl},
+#endif
     {"blf", BPyInit_blf},
     {"bl_math", BPyInit_bl_math},
     {"imbuf", BPyInit_imbuf},