shaderir: Bug fix: Implement atan2 for Metal correctly

Fixes #1360

internal/shader/testdata/atan.expected.metal

@@ -0,0 +1,10 @@
+void F0(float l0, float l1, thread bool& l2);
+
+void F0(float l0, float l1, thread bool& l2) {
+	float l3 = float(0);
+	float l4 = float(0);
+	l3 = atan((l1) / (l0));
+	l4 = atan2(l1, l0);
+	l2 = (l3) == (l4);
+	return;
+}

internal/shader/testdata/atan.expected.vs

@@ -0,0 +1,10 @@
+void F0(in float l0, in float l1, out bool l2);
+
+void F0(in float l0, in float l1, out bool l2) {
+	float l3 = float(0);
+	float l4 = float(0);
+	l3 = atan((l1) / (l0));
+	l4 = atan(l1, l0);
+	l2 = (l3) == (l4);
+	return;
+}

internal/shader/testdata/atan.go

@@ -0,0 +1,7 @@
+package main
+
+func Foo(x, y float) bool {
+	a := atan(y / x)
+	b := atan2(y, x)
+	return a == b
+}

internal/shaderir/glsl/type.go

@@ -65,6 +65,8 @@ func basicTypeString(t shaderir.BasicType) string {
 
 func builtinFuncString(f shaderir.BuiltinFunc) string {
 	switch f {
+	case shaderir.Atan2:
+		return "atan"
 	case shaderir.Dfdx:
 		return "dFdx"
 	case shaderir.Dfdy:

internal/shaderir/program.go

@@ -220,6 +220,7 @@ const (
 	Asin        BuiltinFunc = "asin"
 	Acos        BuiltinFunc = "acos"
 	Atan        BuiltinFunc = "atan"
+	Atan2       BuiltinFunc = "atan2"
 	Pow         BuiltinFunc = "pow"
 	Exp         BuiltinFunc = "exp"
 	Log         BuiltinFunc = "log"
@@ -271,6 +272,7 @@ func ParseBuiltinFunc(str string) (BuiltinFunc, bool) {
 		Asin,
 		Acos,
 		Atan,
+		Atan2,
 		Pow,
 		Exp,
 		Log,