Cleanup: GPU: Move towards an explicit Blend state

This make use of the GLStateStack functions for:
- `GPU_blend()`
- `GPU_blend_set_func()`
- `GPU_blend_set_func_separate()`

The goal is to unify them using an explicit state setting.
This will remove the need to use obscure blend functions

source/blender/windowmanager/intern/wm_gesture.c

@@ -370,7 +370,6 @@ static void draw_filled_lasso(wmGesture *gt)
     MEM_freeN(pixel_buf);
 
     GPU_blend(false);
-    GPU_blend_set_func(GPU_SRC_ALPHA, GPU_ONE_MINUS_SRC_ALPHA);
   }
 
   MEM_freeN(mcoords);