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

tests/store-int16-2.ispc

@@ -0,0 +1,19 @@
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    uniform unsigned int16 x[2*programCount];
+    for (uniform int i = 0; i < 2*programCount; ++i)
+        x[i] = 0xffff;
+    unsigned int16 val = aFOO[programIndex];
+    if (programIndex & 1)
+        x[2+programIndex] = val;
+    RET[programIndex] = x[1+programIndex];
+}
+
+export void result(uniform float RET[]) {
+    if (programIndex & 1)
+        RET[programIndex] = 65535;
+    else
+        RET[programIndex] = programIndex;
+    RET[0] = 65535;
+}