Blender Git Loki

Fix T39950, part 2.

Warn when a modifier does not support optimal drawing and mask/hidden
parts display.

User report was that hidden parts became visible after adding a new
modifier subdivision. Subdivision was added with the Ctrl+number keys.
First part of the fix was to make sure we add a multires instead of the
subsurf modifier in sculpt mode with the PageUp/Down Ctrl+number keys.

We can't really stop users from having added a subsurf modifier already.
We could be "smart" and convert subsurf to mulrires upon entering sculpt
mode maybe but that can easily backfire - or users may actually want
that. For now, just warn that the modifier won't support Hiding/Masking
and optimal drawing.