Make composer arrow, with and without markers, better fill the bounding box area

This patch makes the composer arrow better use the bounding box area:
- if arrows are chosen, the tip of the arrow head lies on the bounding box border
- if no markers or svg markers are chosen, the markers are as close as possible to the border, while still preventing drawing artifacts when the arrow is at 45°.

Funded by Sourcepole QGIS Enterprise