Feature: Raster Layer Contour Renderer

This new renderer draws contour lines that are calculated on the fly from the source raster band. It is possible to set the interval of the contour lines and the symbol used for drawing.
In addition there is support for "index contours" - contour lines with higher intervals, typically drawn with a wider line symbol.
If we generate contour lines on input raster blocks with the same size as our output raster block, the generated lines would contain too much detail. This detail can be reduced by the "downscale" factor - this will request lower resolution of the source raster.

This feature was developed by Martin Dobias