Inline variable re-assignments

[laurentlb]

1. Even if we need the variable f, we can inline the last assignment here (since A is not used afterwards):

f+=r*smoothstep(.5,-1.,sunDir.y);
return f;

2. We could get rid of the variable here, by inlining everything:

vec2 A=vec2(s,1.);
A=v(A,vec2(D,1.));
A=v(A,vec2(u,2.));
...
A=v(A,vec2(P,2.));
return A;

One way to detect these patterns is to introduce new variables when there's an assignment, similar to the SSA form (often seen in compilers). By introducing new variable names, detecting which ones are effectively constant, and aggressively inlining, we should be able to perform many optimizations.