DRW: Fix custom engine not being BGL safe

This was caused by unprotected drawing callbacks.
As of 2.91, we require that all python callbacks used for
drawing needs to be safeguarded by `GPU_bgl_end()` to end the
state tracking override.

source/blender/draw/engines/external/external_engine.c

@@ -36,6 +36,7 @@
 
 #include "GPU_matrix.h"
 #include "GPU_shader.h"
+#include "GPU_state.h"
 #include "GPU_viewport.h"
 
 #include "external_engine.h" /* own include */
@@ -243,6 +244,8 @@ static void external_draw_scene_do(void *vedata)
   type = rv3d->render_engine->type;
   type->view_draw(rv3d->render_engine, draw_ctx->evil_C, draw_ctx->depsgraph);
 
+  GPU_bgl_end();
+
   GPU_matrix_pop();
   GPU_matrix_pop_projection();