First working version with WebGL.

Only the basic things are working and a lot of work still pending. But
first benchmarks (without any optimizations) show a 3-4x increase in
performance!
Fixed rotated textures from TextureAtlas
Fixed Canvas masks

WebGL masks will come next.
Minor code beautification and tweaks
Added shaders and program for primitives

We need this for the WebGL masks.
Added masks for WebGL

Currently only the rectangle and circle masks are implemented. Custom
mask will follow soon and Graphics masks ... later :)
Optimized point transformation
Minor code beautification
Added Polygon class
Added custom mask for WebGL
Minor code beautification

Help dart2js to generate better code. We still have a problem with the
Point class whose x and y properties are not recognized as numbers.
Fixed stencil buffer clear
Fixed Bitmap.clipRectangle
BitmapData feature reincarnations

BitmapData.fromImageElement
BitmapData.sliceIntoFrames
Fixed typo
Minor code beautifications
Breaking change: optional parameter for Stage

But we can specify if the Stage should use WebGL or Canvas.
Disable anitalising - huge performance gain
Assure corrent audio engine initialization
Fixed DisplayObject cache