[glsl-out] Fix atan2 math function

src/back/glsl/mod.rs

@@ -1599,7 +1599,9 @@ impl<'a, W: Write> Writer<'a, W> {
                     Mf::Acos => "acos",
                     Mf::Asin => "asin",
                     Mf::Atan => "atan",
-                    Mf::Atan2 => "atan2",
+                    // glsl doesn't have atan2 function
+                    // use two-argument variation of the atan function
+                    Mf::Atan2 => "atan",
                     // decomposition
                     Mf::Ceil => "ceil",
                     Mf::Floor => "floor",