Regression in rendering with transformation and TextSymbolizer with Halo

A possible regression was found when rendering raster layers that use SLD files extracting contours (using WPS process gs:Contour) and a TextSymbolizer with a Halo.

The screenshots shows artifacts that are rendered on the top left corner of tiles generated by geoserver. These artifacts are only visible if a Halo is set for displaying the values of extracted contours.

Steps to reproduce :

1. Create a GeoTiff store with the given raster.tif

2. Add the two given styles to the created layer

3. Preview the created layer from geoserver with openlayer : use Tiled method for rendering (without tiled, there is only one artifact located on top left corner of the rendered image)

4. One can also add the following parameter to get a better view of the artifacts : &BGCOLOR=0x000000

5. Switch available styles to see that without the halo the artifacts are gone

This bug does not exists in GeoServer 2.7.1.1