This adds sizes 24×24 (common on Linux desktop, for instance in
application bars), as well as 64×64 and 128×128 (common on Linux
mobile).

I kept the existing border sizes, but using the same one from 44×44 to
96×96 sounds a bit weird, it’d probably be best to revisit them at some
point.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>

Preferences that adjust the display, such as Pearl graphics style or
Light Up lit-blobs toggling, shouldn't affect the official icons, even
if a ~/.config/sgt-puzzles exists in the account that builds the
puzzles.

If you configure a Linux build of Puzzles with -fsanitize=address, it
will fail during the icons build, because the icon-maker programs leak
memory when they're run, and by default, this causes ASan to report
all the memory leaks at the end of the program *and then exit 1*.

I don't think 'just fix the memory leaks' is a viable answer. _Some_
of the leaks come from the Puzzles code, and could be fixed by being
more punctilious about freeing everything before exiting (although
that is not necessary for any actually sensible purpose and would
_only_ serve to stop Leak Sanitiser complaining). But some are outside
the Puzzles code completely, apparently from fontconfig. So even if we
fixed all the leaks we could find, we wouldn't prevent the failure
status.

When I want to build with ASan, I've been working around this by
setting ASAN_OPTIONS=detect_leaks=0 in the environment before running
the build command. Easier all round if we just do that _inside_ the
cmake setup.

After Ben fixed all the unwanted global functions by using gcc's
-Wmissing-declarations to spot any that were not predeclared, I
remembered that clang has -Wmissing-variable-declarations, which does
the same job for global objects. Enabled it in -DSTRICT=ON, and made
the code clean under it.

Mostly this was just a matter of sticking 'static' on the front of
things. One variable was outright removed ('verbose' in signpost.c)
because after I made it static clang was then able to spot that it was
also unused.

The more interesting cases were the ones where declarations had to be
_added_ to header files. In particular, in COMBINED builds, puzzles.h
now arranges to have predeclared each 'game' structure defined by a
puzzle backend. Also there's a new tiny header file gtk.h, containing
the declarations of xpm_icons and n_xpm_icons which are exported by
each puzzle's autogenerated icon source file and by no-icon.c. Happily
even the real XPM icon files were generated by our own Perl script
rather than being raw xpm output from ImageMagick, so there was no
difficulty adding the corresponding #include in there.

It wants a 240x130 pixel JPEG.  I've gone for rotating the screenshot
a bit because the store overlays text on the picture and I don't want
horizontal lines in the picture confusing the text.  I think the store
handles dimming the image, so the picture we produce is at full
brightness.

These are built alongside other icons as part of the GTK build.  It
builds new icon sizes of 44 and 88 pixels and then uses ImageMagick to
round off the corners and add a shadow in accordance with the KaiOS
design guide.

This might be useful for creating a KaiOS app of it.  It also
conveniently ensures that Null Game doesn't just build but also runs
correctly.

The square.pl script removed all pixels that were the same colour as the
edge ones, even if that meant removing all of the pixels.  Now it stops
removing pixels at 1x1 so that there will be something left for
ImageMagick to work on.