Map Algebra operations produce different results depending on the projection

[Marc-AndreDaviault]

The result of map algebra functions such as Fslope and Fhillshade depend on the unit of the projection provided in GetMap or GetCoverage requests, for example results for Fslope and Fhillshade are different when requested in 4326 (degrees) and 3979 (metres).
The expected behaviour would be to provide consistent result in every projection.
On the left, you see the result of the WMS with 4326 in the GetMap and on the right it's with 3979 and reprojected by Qgis.