Add support for forward declarations of structures.

Now a declaration like 'struct Foo;' can be used to establish the
name of a struct type, without providing a definition.  One can
pass pointers to such types around the system, but can't do much
else with them (as in C/C++).

Issue #125.

tests/struct-forward-decl-2.ispc

@@ -0,0 +1,36 @@
+
+export uniform int width() { return programCount; }
+
+struct Foo;
+
+void bing(Foo * uniform);
+
+struct Foo {
+    int i;
+    varying float f;
+    Foo * uniform next;
+};
+
+void bar(Foo * uniform f) {
+    bing(f);
+}
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform Foo fa, fb;
+    fa.next = &fb;
+    fb.f = aFOO[programIndex]; 
+    fb.i = 100;
+    bar(&fa);
+    RET[programIndex] = fb.f; 
+}
+
+
+void bing(Foo * uniform f) {
+    f = f->next;
+    f->f *= 2;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2 + 2*programIndex;
+}

tests/struct-forward-decl.ispc

@@ -0,0 +1,33 @@
+
+export uniform int width() { return programCount; }
+
+struct Foo;
+
+void bing(varying Foo * uniform);
+
+struct Foo {
+    float f;
+    int i;
+};
+
+void bar(varying Foo * uniform f) {
+    bing(f);
+}
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    Foo f;
+    f.f = aFOO[programIndex]; 
+    f.i = programIndex;
+    bar(&f);
+    RET[programIndex] = f.f; 
+}
+
+
+void bing(varying Foo * uniform f) {
+    f->f *= 2;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2 + 2*programIndex;
+}