Massive documentation update; fixes many issues. ...

Issues fixed by these documentation updates:

- #1973, fixed filter and BitmapText property docs
- #1908, fixed type reference links
- #1856, fixed name of MeshShader

Mesh interaction

Depecrate `PIXI.math` namespace ...

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

fix typos

Remove the rest of internal module coupling between packages

minor doc changes

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

Fix Mesh Rendering to enable pixi-spine Mesh to render properly

fixed extends doc issue

Merge pull request #1766 from gprzybylowicz/MeshRenderer_fix ...

update indices data before drawing

set _textuteUvs property in constructor

Rope supports textures from spritheet

update indices data before drawing

fix const usages in plugins

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

render methods are private

minor doc cleanup

Fix all the remaining jshint stuff

Moved the mesh folder to the root