ETNA_MESA_DEBUG=no_supertile breaks OpenGL apps

With current Mesa version, most of the games I tried appear broken with ETNA_MESA_DEBUG=no_supertile, which on the other hand is necessary for phoc to function correctly. Ideally this variable would apply only to phoc and not any other application.

An example of artefacts appearing with no_supertile option (it works fine with ETNA_MESA_DEBUG unset):