recreate presentwindows grids from desktopgrid

Theory:
----------
because PresentWindowsEffect::screenCountChanged() is shortcut
for "if (!isActive())", but the desktopgrid doesn't call
PresentWindowsEffect::setActive (or at least
PresentWindowsEffect::screenCountChanged), so the effect can
"miss" the increasing screen count change (it sees the signal,
but ignores it) and when desktopgrid calls it, it assumes the
m_gridSizes array is big enough (but it isn't)

Steps:
----------
1. effects are loaded, 1 screen present
2. 2nd screen gets added, but inactive effects ignore that
3. desktop grid gets activated, updates according to screen count,
   calls presentwindows for screen #2
4. presentwindows data is only prepared for one screen from step 1
   => BOOM

BUG: 351724
CCBUG: 326032
FIXED-IN: 5.4.2
REVIEW: 124960