Some changes mainly include colorful shadows and enhanced preview experience

[RubiTree]

I just make two small change to this library:

1. Bitmap.Config is ALPHA_8 before, this will make shadow color invalid, so I change it to ARGB_4444, and allow use opaque color draw shadow.
2. Cause shadow layer can not preview in xml, so I directly set the background shadow color in edit mode.

[xuminmin0405]

在recyclerview中，如果阴影布局过大，都会造成卡顿。