WindowServer: In HighDPI mode, load high-res window buttons and high-res cursors

Bitmap::load_from_file("foo.png", 2) will now look for "foo-2x.png" and
try load that as a bitmap with scale factor 2 if it exists. If it
doesn't, it falls back to the 1x bitmap as normal.
Only places that know that they'll draw the bitmap to a 2x painter
should pass "2" for the second argument.

Use this new API in WindowServer for loading window buttons and
cursors.

As a testing aid, ctrl-shift-super-i can force HighDPI icons off in
HighDPI mode. Toggling between low-res and high-res icons makes it easy
to see if the high-res version of an icon looks right: It should look
like the low-res version, just less jaggy.

We'll likely have to grow a better API for loading scaled resources, but
for now this suffices.

Things to check:
- `chres 640 480` followed by `chres 640 480 2` followed by
  `chres 640 480`
- window buttons in window context menu (in task bar and on title bar)
  still have low-res icons
- ctrl-shift-super-i in high-res mode toggles sharpness of window
  buttons and of arrow cursorf
- arrow cursor hotspot is still where you'd expect

Userland/Services/WindowServer/Compositor.cpp

@@ -713,7 +713,10 @@ bool Compositor::set_resolution(int desired_width, int desired_height, int scale
         return false;
     }
 
+    int old_scale_factor = Screen::the().scale_factor();
     bool success = Screen::the().set_resolution(desired_width, desired_height, scale_factor);
+    if (success && old_scale_factor != scale_factor)
+        WindowManager::the().reload_icon_bitmaps_after_scale_change();
     init_bitmaps();
     invalidate_occlusions();
     compose();