PolyImage - Support Polygon Sprite in UGUI Image

PolyImage adds support Polygon Sprite in UGUI Image. This can
- save pixels to draw
- pack more sprites into one sheet
- optimize memory and performance at the same time!
- supports 5.x

As you can see, the right column has much less pixels drawed and only a few vertices added:


Also we can pack more sprites together (the camera icon is packed INTO the ring):

Last but not least, it is incredibly easy to use... and no scripts need to be modified (Because it is a subclass of Image)


Feel free to contact me if you encounter any bug or have some feature requests: PolyImage

Thanks @CodeAndWeb for excellent work of TexturePacker Importer.
Thanks @qiankanglai for the idea of UGUI Image with polygon sprites. And this asset has several improvements:
- Rather than using Modifier, subclassing is easy to use and avoid some performance issue
- Add support for Slice mode (which is a lot harder than Simple). The left two modes are still testing.