未完成，由于开了硬件加速 drawBitmapMesh的参数colors在API18以下是不起作用的，所以API18以下的系统上是看不到窗帘的阴影的，正在解决这个问题
according to official api guides, the parameter 'colors' of canvas.drawBitmapMesh won't work when api level lower than 18, so shadow effect is not available when api level lower than 18
阴影问题已解决
shadow issue solved.
无法显示阴影时的效果:
this's what it looks like when shadow effect is not available
显示出阴影时的效果(由于gif对颜色的压缩，阴影效果显示不是很好):
this's what it looks like when shadow effect works
静态效果
