This is needed for Rockbox's polygon drawing.

The Rockbox frontend allows games to be displayed in a "zoomed-in"
state targets with small displays. Currently we use a modal interface
-- a "viewing" mode in which the cursor keys are used to pan around
the rendered bitmap; and an "interaction" mode that actually sends
keys to the game.

This commit adds a midend_get_cursor_location() function to allow the
frontend to retrieve the backend's cursor location or other "region of
interest" -- such as the player location in Cube or Inertia.

With this information, the Rockbox frontend can now intelligently
follow the cursor around in the zoomed-in state, eliminating the need
for a modal interface.

This too seems to have made no difference: each of the commands
    unequal --generate 1000 6dr#12345
    unequal --generate 1000 6adr#12345
delivers the same list of puzzles before and after the fix.

I've only just realised that there's a false-positive bug in the
latin.c solver framework.

It's designed to solve puzzles in which the solution is a latin square
but with some additional constraints provided by the individual
puzzle, and so during solving, it runs a mixture of its own standard
deduction functions that apply to any latin-square puzzle and extra
functions provided by the client puzzle to do deductions based on the
extra clues or constraints.

But what happens if the _last_ move in the solving process is
performed by one of the latin.c built-in methods, and it causes a
violation of the client puzzle's extra constraints? Nothing will ever
notice, and so the solver will report that the puzzle has a solution
when it actually has none.

An example is the Group game id 12i:m12b9a1zd9i6d10c3y2l11q4r . This
was reported by 'groupsolver -g' as being ambiguous. But if you look
at the two 'solutions' reported in the verbose diagnostics, one of
them is arrant nonsense: it has no identity element at all, and
therefore, it fails associativity all over the place. Actually that
puzzle _does_ have a unique solution.

This bug has been around for ages, and nobody has reported a problem.
For recursive solving, that's not much of a surprise, because it would
cause a spurious accusation of ambiguity, so that at generation time
some valid puzzles would be wrongly discarded, and you'd never see
them. But at non-recursive levels, I can't see a reason why this bug
_couldn't_ have led one of the games to present an actually impossible
puzzle believing it to be soluble.

Possibly this never came up because the other clients of latin.c are
more forgiving of this error in some way. For example, they might all
be very likely to use their extra clues early in the solving process,
so that the requirements are already baked in by the time the final
grid square is filled. I don't know!

Anyway. The fix is to introduce last-minute client-side validation:
whenever the centralised latin_solver thinks it's come up with a
filled grid, it should present it to a puzzle-specific validator
function and check that it's _really_ a legal solution.

This commit does the plumbing for all of that: it introduces the new
validator function as one of the many parameters to latin_solver, and
arranges to call it in an appropriate way during the solving process.
But all the per-puzzle validation functions are empty, for the moment.

Another thing I spotted while trawling the whole source base was that
a couple of games had omitted 'static' on a lot of their internal
functions. Checking with nm, there turned out to be quite a few more
than I'd spotted by eye, so this should fix them all.

Also added one missing 'const', on the lookup table nbits[] in Tracks.

This is the main bulk of this boolification work, but although it's
making the largest actual change, it should also be the least
disruptive to anyone interacting with this code base downstream of me,
because it doesn't modify any interface between modules: all the
inter-module APIs were updated one by one in the previous commits.
This just cleans up the code within each individual source file to use
bool in place of int where I think that makes things clearer.

This commit removes the old #defines of TRUE and FALSE from puzzles.h,
and does a mechanical search-and-replace throughout the code to
replace them with the C99 standard lowercase spellings.

encode_params, validate_params and new_desc now take a bool parameter;
fetch_preset, can_format_as_text_now and timing_state all return bool;
and the data fields is_timed, wants_statusbar and can_* are all bool.
All of those were previously typed as int, but semantically boolean.

This commit changes the API declarations in puzzles.h, updates all the
games to match (including the unfinisheds), and updates the developer
docs as well.

Recent changes introduced a couple of non-C89-compatible mixed
declarations and code.

I somehow missed these instances as well. Oh well.