Add support for int8/int16 types. Addresses issues #9 and #42.

tests/gather-int16.ispc

@@ -0,0 +1,13 @@
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    uniform int16 x[programCount];
+    x[programIndex] = programIndex;
+    int a = aFOO[programIndex]-1;
+    unsigned int16 v = x[a];
+    RET[programIndex] = v;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programIndex;
+}