Added symbols size handling in WMS GetLegendGraphic

GEOS-5592

This patch handles size for symbols used in PointSymbolizer for the GetLegendGraphic WMS operation allowing:
 * symbols bigger than icon size to always being drawn inside the icon box
 * have proportional size rendering for symbols used in different rules of the same SLD
 * specify a minimum rendered symbol size using the new minSymbolSize LEGEND_OPTIONS key