Fix "Texture size is zero" error

Error repeat requirements:
1. Use `Texture.empty(width, height)`. (`width > 0` and `width < 1`)
2. Starling.contentScaleFactor < 1
So, `var origWidth:int = width * Starling.contentScaleFactor`, `origWidth` will be `0`. And `flash.display3D::Context3D/createRectangleTexture` throw `ArgumentError: Error #3681: Texture size is zero.`