Add rotation member to QgsMapCanvasMap, use for handling paint event

Fixes the regression with pre-rendering user feedback for
non-rotated maps. The glitch is still there for rotated maps
but that's not a regression.

See http://hub.qgis.org/issues/11811
Simplify the change by not modifing interfaces

.. I've found a link from gsMapCanvasMap and the QgsMapToPixel class
Drop special handling of rotation from MapCanvasMap::paint

Stop assuming absence of rotation from MapCanvasItem::setRect,
but rather assume that the passed rectangle purely encodes
origin and size by only looking at upperLeft corner (xmin,ymax)
and width/height of it, scaled by current map resolution.

Makes the code simpler (maybe) and still fixes the regression.
Also fixes glitches on zoom while map is rotated.

Makes glitches on rotation worst, but again that's not a real
regression.
Further drop now-unused code added with map rotation support

Also add some documenting comments
Merge pull request #1734 from strk/zoom-by-wheel-glitch2

Fix zoomwheel regression (temporary hack)