Depecrate `PIXI.math` namespace ...

Access math members from PIXI when outside of core, require the math module directly when inside of core.

fix issue where refresh could be called too early

don't run mesh ctor until after setting up rope properties

optimize texture property access to bypass the getter

fix rendering problems with mesh and rope

fixed extends doc issue

set _textuteUvs property in constructor

Rope supports textures from spritheet

Move items into their namespace ...

This commit also:

 - fixes some dangling whitespace,
 - fixes some incorrect docs in extras/mesh,
 - moves Text to core
 - moves BitmapText to extras
 - moves math constants to const.js

Moved the mesh folder to the root