Each predicated instruction will be wrapped in an `if (predicate) { instruction_body; }` in the GLSL, where `predicate` is one of the predicate boolean variables previously set by fsetp.

Predicate variables are now added to the generated shader code in the form of 'pX' where X is the predicate id.
These predicate variables are initialized to false on shader startup and are set via the fsetp instructions.

TODO:

* Not all the comparison types are implemented.
* Only the single-predicate version is implemented.