[WIN32SS] Improve Drawing Scrollbars

[WIN32SS] Improve Drawing Scrollbars

A very nice patch of JIRA user "I_kill_Bugs". Many many Thanks!

It addresses:
- CORE-14755 fixed, flashing scrollbar triangles (we know 131 affected apps just from rapps!)
- CORE-13931 fixed, FamiTracker invisible about-dlg
- CORE-14685 improves a bit, but is not entirely fixed
- CORE-11561 improves a bit, but is not entirely fixed
- The patch avoids unnecessary redraws, speeds up GUI interaction and NSIS install times

Jim Tabor had no complains about it, I just did some white-space-tweaks on EOL and indentation.

FTR A testbot run (not on master but on 0.4.13-RC-48-g818e5bc)
https://reactos.org/testman/compare.php?ids=71645,71666 VBox LGTM
https://reactos.org/testman/compare.php?ids=71646,71667 KVM LGTM

I felt tempted to port back, but decided to play safe and commit to master just.