With this, Writer no longer has leftover 1-pixel-width/height lines on
scrolling at the right/bottom of the SwEditWin area.

The problem was that PaintBuffer() painted one less row/column of pixels
than intended. This happened because Rectangle::GetSize() uses
GetWidth() and GetHeight(), which return "bound2 - bound1 + 1", but
because the map mode was in twips, the +1 had no effect.

For example, if top=127 and bottom=762 in pixels, then the needed height
is 636, but (assuming e.g. 96 DPI) counting 11430-1905+1 in twips, then
converting to pixels is only 635, so the last row/column is not painted.

Fix the problem by making sure that GetSize() is invoked on a rectangle
that has the size in pixels, that's how e.g.
SdrPreRenderDevice::OutputPreRenderDevice() uses DrawOutDev(), too.

Change-Id: I6f8686e0a91ba402db93982c25be76992c260abe
(cherry picked from commit b3e645cd)