This fixes a crash where if you switched to a theme that has hover
icons for title buttons, then back to a theme that does not. Then
when you next hover over the title buttons the window server would
crash.
This was due to the hover_bitmap multi-scale bitmap pointer being
non-null, but not containing any bitmaps, so hitting an assertion
when painting.
An Overlay is similar to a transparent window, but has less overhead
and does not get rendered within the window stack. Basically, the area
that an Overlay occupies forces transparency rendering for any window
underneath, which allows us to render them flicker-free.
This also adds a new API that allows displaying the screen numbers,
e.g. while the user configures the screen layout in DisplaySettings
Because other things like drag&drop or the window-size label are not
yet converted to use this new mechanism, they will be drawn over the
screen-number currently.
Since MultiScaleBitmaps only loads icons for the scales in use, we need
to unconditionally reload them so that we pick up the correct bitmaps
for a scale that hasn't been previously used.
This enables rendering of mixed-scale screen layouts with e.g. high
resolution cursors and window button icons on high-dpi screens while
using lower resolution bitmaps on regular screens.
