This drops the bPaintEnd optimization for vertical and horizontal
lines on Windows, where Polyline and LineTo exclude painting the
last pixel of the line. Instead we just always set the last pixel.

It also merges the various "SetPixel" call sites into a common
drawPixelImpl function.

Change-Id: I01cc3c01c908ba74f7978fa90eaaf8d88f923ae3
Reviewed-on: https://gerrit.libreoffice.org/43939Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>