Revision 1866

Several rendering changes:

 - Render the selection into a separate bitmap so that only that
   bitmap needs to be redrawn when the selection changes

 - Render incrementally showing previews and allowing interaction
   before rendering is complete

 - Update the renderer interface a bit. Most parameters of
   RenderMap are now parameters of the constructor

* Thuban/UI/baserenderer.py (BaseRenderer.__init__): Add the map
and the update region as parameters. Update the doc-string
(BaseRenderer.render_map_incrementally): New. Generator function
to renders the map incrementally
(BaseRenderer.render_map): Remove the map argument (it's now in
the constructor) and simply iterate over the
render_map_incrementally generator to draw the map.
(BaseRenderer.draw_shape_layer_incrementally)
(BaseRenderer.draw_shape_layer): Renamed to
draw_shape_layer_incrementally and changed into a generator that
yields True every 500 shapes. Used by render_map_incrementally to
render shape layers incrementally

* Thuban/UI/renderer.py (ScreenRenderer.RenderMap): Removed the
map and region parameters which are now in the constructor
(ScreenRenderer.RenderMapIncrementally): New. Public frontend for
the inherited render_map_incrementally.
(BaseRenderer.draw_shape_layer): Removed.
(ScreenRenderer.draw_selection_incrementally): New. The selection
drawing part of the removed draw_shape_layer as a generator
(ScreenRenderer.layer_shapes): Update because of the region
parameter change
(ExportRenderer.__init__): New. Extend the inherited constructor
with the destination region for the drawing
(ExportRenderer.RenderMap): Removed the map and region parameters
which are now in the constructor

* Thuban/UI/view.py (MapCanvas.PreviewBitmap): New. Return a
bitmap suitable for a preview in a tool
(CanvasPanTool.MouseMove): Use the PreviewBitmap method to get the
bitmap
(MapPrintout.draw_on_dc): Adapt to new renderer interface
(MapCanvas.OnPaint): Handle drawing the selection bitmap if it
exists
(MapCanvas.OnIdle): Update the logic to deal with incremental
rendering and the selection bitmap
(MapCanvas._do_redraw): Handle the instantiation of the render
iterator and the redraws during rendering
(MapCanvas._render_iterator): New. Generator to incrementally
redraw both bitmaps
(MapCanvas.Export): Adapt to new renderer interface.
(MapCanvas.full_redraw): Reset the selection bitmap and the
renderer iterator too
(MapCanvas.redraw_selection): New. Force a redraw of the selection
bitmap
(MapCanvas.shape_selected): Only redraw the selection bitmap