* impl texture2d using imageasset

* texture-cube

* fix issue comment

* remove initWithElement and initWithData; fix issue comment

* define TextureUpdateOpts in gfx/texture-2d.js

* optimize some `new ImageAsset`; optimize image-asset; add updateMipmaps() to Texture2D

* use onLoaded, remove hack

* default material use phone effect;fix skinning model bug