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.
