drm显示下，默认用的BGRA8888颜色格式，代码中无法用alpha绘制透明

[Sokyx]

代码如下：
static ret_t _home_unlock_on_paint_event(void* ctx, event_t* evt)
{
widget_t* canvas_widget = WIDGET(ctx);
canvas_t* canvas = paint_event_cast(evt)->c;
point_t p = {0};

p.x = canvas_widget->x;
p.y = canvas_widget->y;
widget_to_local(canvas_widget,&p);	//transform to widget local xy
int w = canvas_widget->w;
int h = canvas_widget->h;

canvas_set_fill_color(canvas, color_init(0x00, 0xFF, 0x00, 0x00));
canvas_clear_rect(canvas, p.x, p.y, w, h);

return RET_OK;

}
没有效果。界面会显示绿色