Gamepad: Parse SDL_GAMECONTROLLERCONFIG env after default mappings

Addresses comment https://github.com/godotengine/godot/issues/21918#issuecomment-458764923.

main/input_default.cpp

@@ -727,9 +727,17 @@ InputDefault::InputDefault() {
 
 	fallback_mapping = -1;
 
+	// Parse default mappings.
+	{
+		int i = 0;
+		while (DefaultControllerMappings::mappings[i]) {
+			parse_mapping(DefaultControllerMappings::mappings[i++]);
+		}
+	}
+
+	// If defined, parse SDL_GAMECONTROLLERCONFIG for possible new mappings/overrides.
 	String env_mapping = OS::get_singleton()->get_environment("SDL_GAMECONTROLLERCONFIG");
 	if (env_mapping != "") {
-
 		Vector<String> entries = env_mapping.split("\n");
 		for (int i = 0; i < entries.size(); i++) {
 			if (entries[i] == "")
@@ -737,12 +745,6 @@ InputDefault::InputDefault() {
 			parse_mapping(entries[i]);
 		}
 	}
-
-	int i = 0;
-	while (DefaultControllerMappings::mappings[i]) {
-
-		parse_mapping(DefaultControllerMappings::mappings[i++]);
-	}
 }
 
 void InputDefault::joy_button(int p_device, int p_button, bool p_pressed) {