Fix glitches in page outline drap shadow rendering.

* Fixed the small corner areas outside the outline that don't
belong to the drop shadow being filled in with a different color
than the viewport outside the page rects, causing well-visible
artifacts.

* Fixed the drop shadow not being blended against the colorscheme-
aware viewport background color but rather the fixed value of
Qt::gray.

* Made the drop shadow a tad lighter overall to preserve the same
visual result, seeing as how the default color scheme leads to
a darker viewport background color than Qt::gray.