Add foreach_active iteration statement.

Issue #298.

tests/foreach-active-3.ispc

@@ -0,0 +1,16 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    RET[programIndex] = a; 
+
+    if (programIndex & 1)
+        foreach_active (i)
+            ++RET[i];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (1 + programIndex) + ((programIndex & 1) ? 1 : 0);
+}