PhotoDemon-nightly: Adjustments > Brightness and Contrast: fix potential overflow error

Relates to #452 .  Thank you to @opsimathic for catching and reporting!

Brightness and contrast are possibly the oldest adjustments in PhotoDemon (20+ years old at this point), making this a very old bug!

When calculating average luminance for sampled contrast, I had mistakenly typed integer divide instead of floating-point divide and this can produce an overflow for images whose net brightness exceeds MAX_INT (~4.2e9).

Problem now solved; updated nightly incoming shortly...

v9.0: Fix selection tools mistakenly "grabbing" last interactive...

...point when creating many selections in quick succession.

v8.0: Final (I hope) build for 8.0

LET'S DO THIS.

(Final to-do: tagging the release and pointing the README and photodemon.org at it!)

v8.0-beta.1

Finishing touches for the 8.0 beta release

v7.0: Final 7.0 update

After 2.5 years of work, PhotoDemon 7.0 is now available at
photodemon.org.

My endless thanks to everyone who contributed to this release.  May
future releases be much, much faster and easier than this one.  ;)