fix: use explicit PixelFormat::ARGB8888 in screenshot and fix read_pixels

- Update screenshot.rb to explicitly specify SDL2::PixelFormat::ARGB8888
  instead of relying on format=0 default, ensuring consistent byte order
  across all platforms (including Wayland/Linux)
- Update ruby-sdl2 submodule to include read_pixels fixes:
  - Accept SDL2::PixelFormat objects (not just Integer)
  - Remove incorrect ARGB8888 default override
  - Query renderer info for correct pitch when format=0

Addresses feedback from ohai/ruby-sdl2#29.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: takaokouji

ruby/ruby-sdl2

@@ -19,8 +19,8 @@ def maybe_save_screenshot
           return unless @screenshot_path && !File.symlink?(@screenshot_path)
 
           require "smalruby3/smalruby3_imageutil"
-          # read_pixels returns ARGB8888 (little-endian: BGRA bytes)
-          argb_data = @sdl_renderer.read_pixels(nil, 0)
+          # Explicitly request ARGB8888 for consistent byte order across platforms
+          argb_data = @sdl_renderer.read_pixels(nil, SDL2::PixelFormat::ARGB8888)
           # Convert BGRA → RGBA for PNG encoding
           rgba_data = convert_bgra_to_rgba(argb_data)
           Smalruby3::ImageUtil.save_png(rgba_data, @width, @height, @screenshot_path)