Added tests and documentation for soa<> rate qualifier.

tests/soa-11.ispc

@@ -0,0 +1,15 @@
+
+struct Point { float x, y, z; };
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    soa<4> Point pts[2] = { { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } },
+                            { { 13, 14, 15, 16 }, { 17, 18, 19, 20, }, { 21, 22, 23, 24 } } };
+
+    RET[programIndex] = pts[1].y;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 6;
+}