Fix bugs with offsetting for varying values with gathers/scatters.

Fixes issue #134.

tests/gather-struct-vector.ispc

@@ -0,0 +1,31 @@
+
+struct Ray {
+    float<3> v;
+};
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    Ray r[programCount];
+    for (uniform int i = 0; i < programCount; ++i) {
+        r[i].v.x = 100*i + programIndex;
+        r[i].v.y = 200*i + 2*programIndex;
+        r[i].v.z = 300*i + 3*programIndex;
+    }
+
+    Ray *rp = &r[programIndex/2];
+    RET[programIndex] = rp->v.z; 
+}
+
+export void result(uniform float RET[]) {
+    uniform int d0 = 0;
+    uniform int d1 = 0;
+    for (uniform int i = 0; i < programCount; i += 2) {
+        RET[i] = d0+d1;
+        d1 += 3;
+        RET[i+1] = d0+d1;
+        d0 += 300;
+        d1 += 3;
+    }
+}