gl_shader_decompiler: Rework GLSL decompiler type system

GLSL decompiler type system was broken. We converted all return values
to float except for some cases where returning we couldn't and
implicitly broke the rule of returning floats (e.g. for bools or bool
pairs).

Instead of doing this introduce class Expression that knows what type a
return value has and when a consumer wants to use the string it asks for
it with a required type, emitting a runtime error if types are
incompatible.

This has the disadvantage that there's more C++ code, but we can emit
better GLSL code that's easier to read.