[sailfish-browser] Move MOZ_LAYERS_PREFER_OFFSCREEN to browser side. Contibutes to JB#27561
[sailfish-browser] Move MOZ_LAYERS_PREFER_OFFSCREEN to browser side. Contibutes to JB#27561

Intermediate step so that MOZ_LAYERS_PREFER_OFFSCREEN can be removed
from embedlite with breaking things.

sailfishos/sailfish-browser,alinelena/sailfish-browser,sailfishos/sailfish-browser,alinelena/sailfish-browser,alinelena/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,alinelena/sailfish-browser