Cleanup: GPU: Use explicit clear value in GPU_clear* commands

This replace `GPU_clear()` by `GPU_clear_color()` and `GPU_clear_depth()`.
Since we always set the clear value before clearing, it is unecessary
to track the clear color state.
Moreover, it makes it clearer what we clear the framebuffer to.

source/blender/editors/space_script/space_script.c

@@ -127,7 +127,6 @@ static void script_main_region_draw(const bContext *C, ARegion *region)
 
   /* clear and setup matrix */
   UI_ThemeClearColor(TH_BACK);
-  GPU_clear(GPU_COLOR_BIT);
 
   UI_view2d_view_ortho(v2d);