Flatpak repositories can store successive builds (up to 20 in the
Flathub repository in particular), even though by default it installs
only the last build.
I added in `debugging-tips.txt` the commands to search for and
explicitly install an older build of our official GIMP build. This can
be useful to compare some behaviorial changes as reported by users,
without having to re-compile old commits just for a quick test.
Add devel-docs/performance-logs/performance-logs.md, which
describes how to record and view performance logs, and how to
report perofrmance-related issues.
This will be a neat reminder for some of the hidden environment
variables helpful to debug GIMP in some cases, as well as some other
tricks, which we often have to tell people or even which we forget (or
don't even know sometimes) ourselves.
This will be less annoying to just remember to check this file if we
forget the name of some environment variable than to try and grep the
git log or the code.
If someone adds another environment variable which changes some behavior
of the code, please add it here. Also if you know some other helpful
tricks, share with all. Thanks.
