This is a lot easier than faffing about setting up a dialog box
ourself, and also avoids direct access to GtkDialog's action area
(deprecated in GTK 3.16).
Highlight clues of value n in Towers if its row/column contains an
increasing sequence of length n, the last number of which is not equal
to the number of rows/columns (i.e. such that the sequence will have
to be extended, in violation of the clue).
Commit 8b491946e had a bug: configure_area stopped doing most of its
work if the new size already matched fe->w and fe->h, but in fact the
GTK2 resize_fe() _already_ set up fe->w and fe->h for the new size. I
managed not to notice, because I checked it all worked on GTK 3 but
only tested resizing to a _smaller_ puzzle on GTK 2. Ahem.
Now we don't change fe->w and fe->h at all until configure_area is
called. Also, we initialise them to dummy values at setup time, so
that configure_area won't compare the new size with uninitialised
data.
Draw pencil-mode cursor before corners of thick lines in order to make
the thick lines appear on top of the north-west corner of the pencil
cursor, just like they do with the non-pencil cursor.
Keyboard dragging while holding Control now moves the cursor to the
target square. Shift-Control-arrowkey performs the previous behavior
of Control-arrowkey.
Its absence was breaking the icon build on Ubuntu 14.04, because you
ask convert to map an image into a specific 16-colour palette, and it
does so and _then_ nonconsensually maps those colours in turn through
a colourspace transformation you didn't ask for, causing icon.pl to
fail an assertion when it finds an RGB value not in the palette.
I think I did this in GTK2 on the basis that our server-side cache
pixmap was double-buffering enough for us - any puzzle which erased a
big area with a background rectangle and then redrew over the top of
it would do so only on the off-screen pixmap, and the updates would
only be copied to the visible window after it was all done.
In GTK3, I don't think there's any need - this is all the usual way
things are done anyway, as far as I can see. So I've turned this call
back off, at least until I hear of a reason why I need it again.
