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.

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.

This function gives the front end a way to find out what keys the back
end requires; and as such it is mostly useful for ports without a
keyboard. It is based on changes originally found in Chris Boyle's
Android port, though some modifications were needed to make it more
flexible.

The fields (ui->dragx,ui->dragy) stored the pixel coordinates of the
visible cursor (if any), no matter whether that cursor was being
displayed in response to a mouse dragging action or arrow-key
activity. But this meant that resizing the window while the keyboard
cursor was visible would cause the cursor to be drawn in totally the
wrong place in the newly resized window: you get a new drawstate with
a fresh blitter (so at least no part of the old-size window is
accidentally 'restored' on to the new-size one), but the ui fields
saying where _next_ to draw the cursor would still have bogus values
left over from the previous window size.

To fix this, I've arranged that we simply don't use ui->dragx and
ui->dragy any more in keyboard cursor mode: instead, we leave it to
game_redraw to spot that the keyboard cursor is visible and compute
its pixel coordinates for display.

A knock-on effect is that when we need to know which region is under
the cursor during interpret_move, we can't use the previous strategy
of just calling region_from_coords(ui->dragx, ui->dragy) regardless of
which kind of cursor is active. So I've split that function up into an
inner section taking a tile and a displacement from the tile's centre
and an outer part which derives those from real pixel coordinates in
the case where the cursor is originating from a mouse drag; then
there's a new alternative outer part which derives the same (tile,
displacement) pair purely from the game_ui keyboard cursor data
without having to explicitly translate into pixels and back in the
middle. Probably a less fragile strategy anyway.

I've also added that warning option and -Werror to the build script,
so that I'll find out if I break this property in future.

They're never dynamically allocated, and are almost always string
literals, so const is more appropriate.