This is similar to what is achieved in
`QgsRasterLayer::refreshRenderer()` to refresh the renderer according
to an extent. Contrary to the first one, this method does not perform
any GUI update or emit any signal.
It is not used at the moment. This will replace the logic to refresh a
renderer in the following commits.
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.
