WMS GetLegendGraphic should honour class->scale

[dmorissette]

Bug 809 implemented support for the SCALE parameter in GetLegendGraphic, but as
was pointed out in bug 809 comment 11 to 13, this will be of no real use until
msDrawLegend() honours the minscale/maxscale settings of the classes.

So we're filing this bug as a future enhancement: we should add support for
class minscale/maxscale for GetLegendGraphic requests when SCALE is explicitly
specified. Possible impacts on the mapserv CGI's legend should be taken into
account if/when we do this.

Also make sure that whatever is done in this bug is compatible with the fix to
bug 1007.