Add clearBeforeRender for <webgl>

Maybe "clearBeforeRender" this is not the best naming (since it will clear anyway..), but this aligns canvas and webgl:

Basically it allows the user to do clearColor whenever they want, instead of this being done for them. This is useful in the following circumstance:

1) The game/program loads a background image, but not right away.
2) Before the background image is loaded, we want to fill the canvas with a certain color (for example, white).
3) In canvas mode, this is done by setting ``clearBeforeRender`` to false and then adding a single ``fillRect`` command.
4) In webGL mode, the equivalent is running ``gl.clearColor`` and then ``gl.clear``.
5) We can of course set the background color in webGL mode and this probably won't be expensive (although I don't know), but in canvas it *is* expensive to clear/fill on every frame.
6) Thus, the user would have to detect webGL mode or canvas mode themselves, and either set the background color or not.
7) To eliminate this coding complexity for the user, this small bit of extra code aligns ``clearBeforeRender`` (or, as much as possible) to the canvas implementation.
Reduce some logic redundancy in clearRect/fillRect code for Canvas...?

Topic.
Update CanvasRenderer.js
Update CanvasRenderer.js
Rewrote update transform

matrix now used correctly.
Updated the Matrix class

New functions and a bit of a tidy up
Removed the updateFrames global Array
Removed PIXI.texturesToUpdate

No longer required
Removed update and destroy webGL functions
Created new initContext function for webGLRenderer

less code = good :)
Little blendMode tweak
Fixed Canvas RenderTexture

Now uses matrix also
Fixed pivot issue
Update WebGLRenderer.js
Made RemoveChild no longer returns error
removed texture check from render loop

saved on if statement! better than none :)
Replace `tabs` char on 4 `spaces` chars in examples
Replace `==` on `===` in examples
merge #1018; replace tabs with 4 spaces in examples
Merge branch 'PixxxeL-box2d-example' into dev