D8388: Do not hardcode unhovered windows brightness on present windows effect

Zren added a comment.


  I just realized how difficult "drawing a rectangle" outline is in C++ KWin Effects. We're required to have codepaths for OpenGL, XRender, and QPainter... uhg. No clue how to even test QPainter.
  
  I was able to use the showpaint + snaphelper effects as examples for getting this. I currently draw a rectangle using `GL_LINES` around any window that has `winData->highlight > 0`. I think I'll try only outlining a single highlighted window and only if it's `winData->highlight == 1` after the "zoom in" animation finishes. Now that I'm only outlining 1 window maybe. To animate the outline opacity, I'd need to draw multiple rectangles which is tricky.
  
  F6857292: screen-2019-05-28_18.54.51.mp4 <https://phabricator.kde.org/F6857292>

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D8388

To: salvacorts, #kwin, #vdg, #plasma
Cc: ndavis, ahmadsamir, Zren, muratcileli, zzag, graesslin, ngraham, kwin, #kwin, jraleigh, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart