GEOS-4834: original patch to fix transparency in animaged gif + tests

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
Merge pull request #653 from geosolutions-it/GEOS-4834-2.5.x

GEOS-4834: original patch to fix transparency in animaged gif + tests (backport to 2.5.x)