DrwManager: Remove the need for buffers to be available.

This way we can just avoid binding a texture by setting it's ref to NULL.

source/blender/draw/intern/draw_manager.c

@@ -1733,6 +1733,10 @@ static void draw_shgroup(DRWShadingGroup *shgroup, DRWState pass_state)
 					break;
 				}
 				tex = *((GPUTexture **)uni->value);
+				if (tex == NULL) {
+					/* In case texture is not yet available */
+					break;
+				}
 				GPU_texture_bind(tex, uni->bindloc);
 
 				bound_tex = MEM_callocN(sizeof(DRWBoundTexture), "DRWBoundTexture");