Actually typecheck the arguments to functions called through function pointers.

(Somehow this wasn't being done before.)
Errors are now issued if too few arguments are used when calling through
a function pointer, too many arguments are used, or if any of them can't be
type converted to the parameter type.

tests_errors/fptr-typecheck-1.ispc

@@ -0,0 +1,9 @@
+// Too many parameter values provided in function call
+
+float bar(float a, float b);
+
+export uniform int foo(uniform int x[], uniform int i[]) {
+    float (*fptr)(float, float) = bar;
+//CO    bar(0,1,2);
+    fptr(0., 1, 2);
+}

tests_errors/fptr-typecheck-2.ispc

@@ -0,0 +1,9 @@
+// Can't convert argument of type "void * const uniform" to type "float" for funcion call argument.
+
+float bar(float a, float b);
+
+export uniform int foo(uniform int x[], uniform int i[]) {
+    float (*fptr)(float, float) = bar;
+//CO    bar(0,1,2);
+    fptr(NULL, 1);
+}

tests_errors/fptr-typecheck-3.ispc

@@ -0,0 +1,8 @@
+// Too few parameter values provided in function call (1 provided, 2 expected).
+
+float bar(float a, float b);
+
+export uniform int foo(uniform int x[], uniform int i[]) {
+    float (*fptr)(float, float) = bar;
+    fptr(1.);
+}