Improve the logic about calculate vertices (#3746)
Improve the logic about calculate vertices (#3746)

Reason:
* Do similar things in similar way :  compute left / top , then add width/height .
* A very very very very (...* 1000000) bit little performance improvement :  one `-` & one `*` become one `+`.
* fix some words in document.
sfinktah/pixi.js,ceco-fmedia/pixi.js,ceco-fmedia/pixi.js,Cristy94/pixi.js,bma73/pixi.js,jojuntune/pixi.js,pixijs/pixi.js,jojuntune/pixi.js,lucap86/pixi.js,finscn/pixi.js,bma73/pixi.js,Makio64/pixi.js,pixijs/pixi.js,GoodBoyDigital/pixi.js,sfinktah/pixi.js,englercj/pixi.js,finscn/pixi.js,Makio64/pixi.js,lucap86/pixi.js,themoonrat/pixi.js,themoonrat/pixi.js,staff0rd/pixi.js,staff0rd/pixi.js,Cristy94/pixi.js,englercj/pixi.js