fixed patch from #4834 to fix transparency in animated gif

this is actually the patch from Simone Giannecchini, with one little
fix in RenderedImageMapResponse to make it work (again)

see: https://jira.codehaus.org/browse/GEOS-4834

I'm not aware of tests which can test this automatically, but if possible please guide me and I will try to add tests for this.

I can create a simple shapefile and url's to test this, if needed.  
This work is related to https://jira.codehaus.org/browse/GEOS-6479 for which this shapefile also could be used then.
OR is there already a time/elevation related test shape in the demo data?