Use the `keep` stretch aspect by default

This makes Godot avoid distortion when resizing the window to an aspect
ratio that doesn't match the project's base aspect ratio.

Since this setting has no effect when the stretch mode is `disabled`
(the default), this won't impact projects not using the `canvas_items`
or `viewport` stretch modes.

main/main.cpp

@@ -2193,7 +2193,7 @@ bool Main::start() {
 			//standard helpers that can be changed from main config
 
 			String stretch_mode = GLOBAL_DEF_BASIC("display/window/stretch/mode", "disabled");
-			String stretch_aspect = GLOBAL_DEF_BASIC("display/window/stretch/aspect", "ignore");
+			String stretch_aspect = GLOBAL_DEF_BASIC("display/window/stretch/aspect", "keep");
 			Size2i stretch_size = Size2i(GLOBAL_DEF_BASIC("display/window/size/width", 0),
 					GLOBAL_DEF_BASIC("display/window/size/height", 0));
 
@@ -2252,7 +2252,7 @@ bool Main::start() {
 							"display/window/stretch/mode",
 							PROPERTY_HINT_ENUM,
 							"disabled,canvas_items,viewport"));
-			GLOBAL_DEF_BASIC("display/window/stretch/aspect", "ignore");
+			GLOBAL_DEF_BASIC("display/window/stretch/aspect", "keep");
 			ProjectSettings::get_singleton()->set_custom_property_info("display/window/stretch/aspect",
 					PropertyInfo(Variant::STRING,
 							"display/window/stretch/aspect",