Implemented a new multiple texture batching system (based on ideas in the
advanced batcher by Matt DesLauriers https://github.com/mattdesl/pixi.js/blob/culling/src/pixi/renderers/webgl/utils/WebGLAdvancedBatch.js
but extended to use a sampler array).
Tidied up the shader ownership (now the individual classes responsible for rendering such as batch, graphics etc now own
their own shaders).
Removed the redundant recursion benchmark.