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/generic/CMakeLists.txt

@@ -17,7 +17,6 @@ set(INC_SYS
 )
 
 set(SRC
-  bgl.c
   bl_math_py_api.c
   blf_py_api.c
   bpy_threads.c
@@ -27,7 +26,6 @@ set(SRC
   py_capi_rna.c
   py_capi_utils.c
 
-  bgl.h
   bl_math_py_api.h
   blf_py_api.h
   idprop_py_api.h
@@ -40,6 +38,14 @@ set(SRC
   python_utildefines.h
 )
 
+if(WITH_OPENGL)
+  list(APPEND SRC
+    bgl.c
+
+    bgl.h
+  )
+endif()
+
 set(LIB
   ${GLEW_LIBRARY}
   ${PYTHON_LINKFLAGS}