Update seed_rng() in stdlib to take a varying seed.

Previously, we were trying to take a uniform seed and then shuffle that
around to initialize the state for each of the program instances.  This
was becoming increasingly untenable and brittle.

Now a varying seed is expected and used.

tests/rand-distrib-1.ispc

@@ -3,7 +3,7 @@ export uniform int width() { return programCount; }
 
 export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
     RNGState state;
-    seed_rng(&state, 1);
+    seed_rng(&state, programIndex);
     int count[32];
     for (uniform int i = 0; i < 32; ++i)
         count[i] = (b == 5.) ? 0 : 1;

tests/rand-distrib.ispc

@@ -3,7 +3,7 @@ export uniform int width() { return programCount; }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     RNGState state;
-    seed_rng(&state, 1);
+    seed_rng(&state, programIndex);
     float sum = 0;
     uniform int iters = 40000;
     for (unsigned int i = 0; i < iters; ++i)