GeoPainter: consistent qreal-based API, separate regionFromPixmapRect()

Summary:
GeoPainter API had a mixture of int and qreal-based methods, and internally
was inconsistently relaying to int or qreal-based QPainter methods.

Regions used for painting rects and ellipses depend on whether anti-aliasing
is enabled or not, and can result in offsets of at least 1 pixel.
So calculations now tries to respect that.

GeoPainter::regionFromRect() was used for getting the region of pixmaps
being rendered. Especially in non-anti-aliased mode though this does not
match, also is there no stroke-width on painting pixmaps.
A special GeoPainter::regionFromPixmapRect() should serve that purpose now.

Reviewers: nienhueser, shentey, rahn, #marble

Reviewed By: rahn, #marble

Differential Revision: https://phabricator.kde.org/D2279