internal/shader: Add more tests and improve the comment

Updates #1192

internal/shader/stmt.go

@@ -642,7 +642,7 @@ func (cs *compileState) assign(block *block, fname string, pos token.Pos, lhs, r
 				}
 				t := rhsTypes[i]
 				if t.Main == shaderir.None {
-					// TODO: This is to determine a type when the rhs is a constant,
+					// TODO: This is to determine a type when the rhs values are constants (not literals),
 					// but there are no actual cases when len(lhs) != len(rhs). Is this correct?
 					t = toDefaultType(rhsExprs[i].Const)
 				}

internal/shader/testdata/assign_multiple2.expected.vs

@@ -0,0 +1,14 @@
+void F0(out vec2 l0);
+
+void F0(out vec2 l0) {
+	float l1 = float(0);
+	float l2 = float(0);
+	float l3 = float(0);
+	float l4 = float(0);
+	l2 = 1.0;
+	l1 = l2;
+	l4 = 2.0;
+	l3 = l4;
+	l0 = vec2(l1, l3);
+	return;
+}

internal/shader/testdata/assign_multiple2.go

@@ -0,0 +1,6 @@
+package main
+
+func Foo() vec2 {
+	r1, r2 := 1.0, 2.0
+	return vec2(r1, r2)
+}