Jpeg transparency

Bug: An image with black background color is returned, when requesting a JPEG image and setting transparency to true (GetMap request).
Example request for utah workspace to reproduce the bug: [1].

The fix solves this problem by always using the given background color when requesting a JPEG image, as JPEG does not support transparency.

[1] http://localhost:8080/deegree/services/wms?FORMAT=image%2Fjpeg&TRANSPARENT=true&BGCOLOR=0xffffff&LAYERS=countyboundaries&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&EXCEPTIONS=application%2Fvnd.ogc.se_inimage&SRS=EPSG%3A900913&BBOX=-12914551.936364,4165602.5406961,-11780837.933042,5691897.1212211&WIDTH=927&HEIGHT=1248