Fix T39517,

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.

source/blender/gpu/GPU_buffers.h

@@ -180,7 +180,7 @@ void GPU_update_bmesh_pbvh_buffers(GPU_PBVH_Buffers *buffers,
                               struct GHash *bm_faces,
                               struct GSet *bm_unique_verts,
                               struct GSet *bm_other_verts,
-                              int show_diffuse_color);
+                              bool show_diffuse_color);
 
 void GPU_update_grid_pbvh_buffers(GPU_PBVH_Buffers *buffers, struct CCGElem **grids,
                              const struct DMFlagMat *grid_flag_mats,