Type checking is done either against a literal value (number, string...)
or against the operator in case of an expression.
Sometimes it is not possible to infer only one type, for example
the value 'transparent' could either be a color or a string. This
is covered by the fact that all operators expect exactly one type
for their arguments.
These variables are accessed using the `var` operator, and are meant
to be mutated directly in the style object. When doing so,
the new value will always be used for the render.
This is intended to be used for modifying the style through external
parameters such as an input field, a slider, mouse position etc.
There is now a ShaderBuilder class with chainable methods
to specify more easily the contents of the final shaders.
This is to avoid passing around large objects to different functions,
and allow for a richer API.
The documentation has also been corrected and clarified.
Now two methods are available: `readAll` and `readPixel`,
and the data from the render target is not re-read every time unless
`clearCachedData` is called.
The `WebGLHelper` class now provides a `makeProjectionTransform` method
that updates a transform to match the projection for a given frame state.
This also means that the WebGLHelper does not set the projection matrix
uniform anymore, this is the responsibility of the renderer as
the rendered coordinates will not be in world space from now on.
Now the shader and program caches are simply arrays of native WebGL created objects.
The WebGLHelper simply takes the sources of the frag and vert shader and produces a program.
This removes 2 classes & reduces the general verbosity of the API.
Also a `getShaderCompilationErrors` was added on `WebGLHelper` to help debug GLSL errors.
