Refactor shader source classes, in order to work around a Firefox bug.

The Firefox bug prevents shaders from compiling with the CORDIC algorithm, so we detect Firefox and fall back to the original algorithm.  Issue #2197.

The overall goal of the refactoring is to allow `#defines` to affect built-in functions, which was not possible before.  In order to do this, we need the `#defines` to appear at the top of the source, so I've merged the procedural shader generation previously done in `ShaderProgram` with the generation done in `createShaderSource`.

* `createShaderSource` (private) is removed and replaced by `ShaderSource`. `ShaderSource` now manages an array of `#defines`, concating source shaders, and automatically including built-in functions, keeping everything in the correct order.
* `ShaderCache` (and by extension `Context.createShaderProgram` and `Context.replaceShaderProgram`) now accept either `ShaderSource` objects, or a single string, for vertex and fragment shaders.
* `ShaderProgram` constructor (private) now takes an options object.  It also exposes the vertex and fragment shaders as `ShaderSource` objects instead of strings.  External code can still clone the `ShaderSources` and then screw around with the implementation to make new shaders procedurally, as `OIT` does.
Needing this comment is fishy.  We pretty much have to have implemented the shader generator or got burned to know to do this.  Do you think, instead, we could change the shader generator to only pre-pend if the built-in is not already defined?