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

tests/int16-wrap.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float bb) {
+    unsigned int16 a = aFOO[programIndex], b = bb;
+    RET[programIndex] = ((unsigned int16)4000*a)+b; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (((4000*(programIndex+1))&0xffff)+5)&0xffff;
+}