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.

source/blender/blenkernel/intern/DerivedMesh.c

@@ -1593,7 +1593,7 @@ static void mesh_calc_modifiers(Scene *scene, Object *ob, float (*inputVertexCos
 			continue;
 		}
 		if (sculpt_mode &&
-		    (!has_multires || multires_applied || ob->sculpt->bm))
+			(!has_multires || multires_applied || sculpt_dyntopo))
 		{
 			bool unsupported = false;
 
@@ -1614,6 +1614,9 @@ static void mesh_calc_modifiers(Scene *scene, Object *ob, float (*inputVertexCos
 				modifier_setError(md, "Not supported in sculpt mode");
 				continue;
 			}
+			else {
+				modifier_setError(md, "Hide, Mask and optimized display disabled");
+			}
 		}
 		if (needMapping && !modifier_supportsMapping(md)) continue;
 		if (useDeform < 0 && mti->dependsOnTime && mti->dependsOnTime(md)) continue;