Issue here is that "show diffuse" option does not respect its intended
purpose which is to be used only for masking.
There are a couple of caveats here:
Dyntopo and multires -always- have mask data enabled, and thus as soon
as one goes to dyntopo mode or adds a multires modifier he would get the
default grey color instead.
Matcaps would break when nodes asked for a diffuse material color (this
was broken before too). Solved by adding global material state for when
matcaps are enabled. Also matcaps don't always played well with VBOs
off.
Added a few more missing updates for mask operators to notify
show_diffuse property as changed. This was also needed on rebuilding
dyntopo pbvh.
Also make zero mask color duller again after artist feedback.
03bd418d16