Remove manual pixel conversion in WallpaperImage

I used to manually convert pixels from RGBA=>ARGB because Wayland
compositors are only guaranteed to support XRGB and ARGB, but zigimg
doesn't include either of those. This was a bit slow, especially on
debug builds (though not *super* noticeable on release builds).

I realized, though, that zigimg's Rgba32 format is the same as pixman's
a8b8g8r8... on little-endian. I kept the old code just in case someone
out there happens to be running beansprout on MIPS, but I have not
tested it.

src/Output.zig

@@ -457,7 +457,7 @@ pub fn renderWallpaper(output: *Output) !void {
     }
     // Scale our loaded image and then copy it into the Buffer's pixman.Image
     const wallpaper_image = context.wallpaper_image orelse return error.MissingWallpaperImage;
-    const image = wallpaper_image.image;
+    const image = wallpaper_image.pix_image;
     const image_data = image.getData();
     const image_width = image.getWidth();
     const image_height = image.getHeight();