Improvements to the canvas vector renderer

[tsauerwein]

This patch addresses several issues with the canvas vector renderer:

• The feature order is now taken into account (last added features are drawn on top). This also fixes #2663.
• Three modes were added: STATIC, INTERACTIVE and RTREE
  • STATIC: Best option if the features should be displayed only once and if there is no further interaction (panning/zooming/selecting features) with the map.
  • INTERACTIVE: Mode optimized for interactions with the map, requires the calculation of the features' bounds which slows down. The feature bounds are used to render only the features that are inside the current map extent and to find the feature that is selected by a mouse click (getFeatureIdFromEvent).
  • RTREE: Like INTERACTIVE, but a R-Tree is used to find features within a bounding-box. Uses the  R-Tree Library for Javascript.
• When panning and zooming the method 'redraw()' is called only once, instead of twice.

[tsauerwein]

Patch

[fredj]

for the double redraw issue, see #2801

[erilem]

See the feature hit detection patch in #2839.