Increase screen resolution to 1280x1024 (60 Hz)

With MAME 0.234, unevenstretchx/y and keepaspect work correctly
together [1], so it's now worthwhile to increase screen resolution
so more of the screen is used for games with unusual aspect ratios.

This is the maximum resolution my arcade monitor supports.

Also, resize splashscreen.png to higher resolution.

[1] mamedev/mame#8209

init-arcade.yml

@@ -488,18 +488,18 @@
     - vt.global_cursor_default=0
     # Rotate the HDMI 1 display 90 degrees
     # Resolution agrees with hdmi_mode below.
-    - video=HDMI-A-1:1024x768M@60,rotate=90
+    - video=HDMI-A-1:1280x1024@60,rotate=90
     # Disable console blank timer
     - consoleblank=0
   - config_txt_changes:
     # Necessary for my VGA arcade monitor (needs HDMI adapter)
     - option: hdmi_force_hotplug
       value: 1
-    # hdmi_group: DMT / hdmi_mode: 1024x768 60 Hz
+    # hdmi_group: DMT / hdmi_mode: 1280x1024 60 Hz
     - option: hdmi_group
       value: 2
     - option: hdmi_mode
-      value: 16
+      value: 35
     # Disable rainbow splash screen on boot
     - option: disable_splash
       value: 1
@@ -679,6 +679,8 @@
     loop:
     - key: autorol
       value: 1
+    - key: autostretchxy
+      value: 1
     - key: pluginspath
       value: /usr/local/games/mame/plugins
     - key: rompath