Some saturation tests fixes

tests/paddus_vi8.ispc

@@ -2,7 +2,7 @@
 export uniform int width() { return programCount; }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
-    varying int8 a = 255, b = 255; // max unsigned int8
+    varying unsigned int8 a = 255, b = 255; // max unsigned int8
     RET[programIndex] = paddus(a, b);
 }