Improve Integer Scaling behavior

...so it always upscales the display from the framebuffer to fit the aspect ratio by stretching either horizontally or vertically instead of always stretching horizontally, which sometimes (actually most of the time) downscales the image

src/frontend-common/fullscreen_ui.cpp

@@ -4506,6 +4506,9 @@ void FullscreenUI::DrawAdvancedSettingsPage()
   DrawFloatRangeSetting(bsi, "Display FPS Limit",
                         "Limits how many frames are displayed to the screen. These frames are still rendered.",
                         "Display", "MaxFPS", Settings::DEFAULT_DISPLAY_MAX_FPS, 0.0f, 500.0f, "%.2f FPS");
+  DrawToggleSetting(bsi, "Always Stretch Display Horizontally",
+                    "Enables old DuckStation behavior for Integer Scaling, where the display is always stretched horizontally.",
+                    "Display", "AlwaysStretchHorizontally", false);
 
   MenuHeading("PGXP Settings");