I was recently informed that unicode has official symbols for
window controls, and I put together a change to use them, which
worked, as long as a suitable font was installed. However, it's
not that hard to get a normal system that lacks an appropriate
font, and libass wants to print warnings if the symbols aren't
in the default font, which will almost always be true.
So, I gave up and added the symbols to the custom osd font that
we already have. This ensures they are always available, and
that they are aligned consistently on all platforms.
I took the symbols from the `symbola` font, as this has a suitable
licence and the symbols look nice enough.
Symbola Licence:
Fonts are free for any use; they may be opened, edited,
modified, regenerated, packaged and redistributed.
Finally, as we now have access to an un-maximize symbol, I added
logic to use it when the window is maximized.
