Replace the somewhat-recently-added "nothing" item in the tool bar
with the "Fill" tool, which has been converted back from a Magic tool
to a regular tool.
Also, change bash "==" to sh "=" in "if [ ... ]" tests in Makefile.
Also, link to math library (via "-lm") to make sure "max()" is available to
magic tools.
Also, add missing mention of "--newcolorslast" and "--newcolorsfirst" to manpage.
Respond to a SIGUSR1 or SIGUSR2 signal by configuring itself
for autosave, saving a new file, or overwriting the current,
respectively, and sending itself a SIGTERM (which _generally_
causes Tux Paint to quit, depending on where you are in the UI).
Closes https://sourceforge.net/p/tuxpaint/feature-requests/188/
Bumping to 0.9.24; going to add a new feature momentarily.
Improved usage output (e.g., that of "tuxpaint --help"); broke
the options into groups that match the tabs in Tux Paint Config.,
added some missing options.
Documenting Mark Kim's recent changes:
* Less CPU-hungry progress bar (during Stamp loading)
* Verbose debugging options
* (Also, mentioned debugging _at all_, in INSTALL.txt)
Bumping version to 0.9.23a, to match the test build he's working
on for macOS.
