More efficient implementation of frandom() in stdlib

tests/rand-distrib.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RNGState state;
+    seed_rng(state, 1);
+    float sum = 0;
+    uniform int iters = 10000;
+    for (unsigned int i = 0; i < iters; ++i)
+        sum += frandom(state);
+    float avg = sum / iters;
+    RET[programIndex] = (avg > 0.495 && avg < 0.505) ? 1 : 0;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}