rgui: don't pre-center custom viewport (fixes right-aligned menu)

Since 439c672 ("Support viewport bias with integer overscale and
custom aspect ratios"), the ASPECT_RATIO_CUSTOM path in
video_viewport_get_scaled_aspect2() / video_viewport_get_scaled_integer()
positions the custom viewport as:

   x  = custom_vp->x;
   padding_x = vp_width - custom_vp->width;   (positive when the menu is
                                               narrower than the screen)
   x += padding_x * vp_bias_x;                (default bias 0.5)

i.e. the driver now centers the custom viewport itself via the viewport
bias. RGUI, however, was *also* baking the centering into the viewport it
hands to the driver:

   vp.x = (full_width - width) / 2;

so the offset was applied twice. On a wide screen with a 4:3 menu aspect
the menu ended up flush against the right edge (and the bottom), and
because RGUI writes these values into the shared settings->video_vp_custom,
opening the menu in-game also shifted the content viewport and left it
shifted afterwards.

Set the base position to 0 and let the driver's bias logic do the
centering (default 0.5 == centered), matching how every other
custom-viewport consumer is now expected to behave. All in-tree video
drivers route their viewport through video_driver_update_viewport(), so
none position RGUI from custom_vp->x directly.

Fixes a regression reported on KMSDRM + GL (Pi4, aarch64); driver-
independent.

Author: libretroadmin

menu/drivers/rgui.c

@@ -6263,8 +6263,15 @@ static void rgui_update_menu_viewport(
       rgui->menu_video_settings.vp.height = 1;
    }
 
-   rgui->menu_video_settings.vp.x = (vp.full_width - rgui->menu_video_settings.vp.width) / 2;
-   rgui->menu_video_settings.vp.y = (vp.full_height - rgui->menu_video_settings.vp.height) / 2;
+   /* Leave the viewport at the origin and let the video driver's
+    * viewport-bias logic centre it (default bias 0.5 == centred).
+    * Pre-centring here as well (vp.x = (full_width - width) / 2)
+    * double-applies the offset since 439c672c22 made the
+    * ASPECT_RATIO_CUSTOM path add padding * bias on top of vp.x,
+    * which pushed the menu hard against the right/bottom edge on
+    * wide screens. */
+   rgui->menu_video_settings.vp.x = 0;
+   rgui->menu_video_settings.vp.y = 0;
 }
 
 static bool rgui_set_aspect_ratio(