lv_layer_t::_clip_area field

NEVER USE IT DRAW UNITS. USED INTERNALLY DURING DRAW TASK CREATION. The current clip area with absolute coordinates, always the same or smaller than `buf_area` Can be set before new draw tasks are added to indicate the clip area of the draw tasks. Therefore `lv_draw_add_task()` always saves it in the new draw task to know the clip area when the draw task was added. During drawing the draw units also sees the saved clip_area and should use it during drawing. During drawing the layer's clip area shouldn't be used as it might be already changed for other draw tasks.