* Fix a bug about compute Text canvas size.

There are 2 bugs :
* should NOT use `this.context.lineWidth`  before `this.context.lineWidth = style.strokeThickness`.
* the default value of `style.strokeThickness` is `0` , but  `this.context.lineWidth` couldn't be 0 ( in HTML5 , context.lineWidth = 0 will be ignore). So we should NOT use `this.context.lineWidth` for computing size.

* Update Text.js

Don't need  ` + style.strokeThickness` again.

* Update Text.js

* Update Text.js

refactor :   change the  codes of computing `canvas.width` & `canvas.height`  in the same code style.

* use  `style` instead of `this._style`