Attempts to set the input band for the renderer.
Returns TRUE if the band was successfully set, or FALSE if the
band could not be set.
This was implemented in various raster renderer subclasses,
but it was necessary to down cast and then call the individual
methods (which don't have consistent names!).
Instead, add a top level virtual method so that it's easy to
change the input band for the renderers.
This new renderer draws contour lines that are calculated on the fly
from the source raster band. It is possible to set interval of contour
lines and symbol used for drawing.
In addition there is support for "index contours" - contour lines
with higher interval, typically drawn with a wider line symbol.
If we generate contour lines on input raster block 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.
