Fix: New brushes don't initialize mask_stencil correctly

These should have a default size/pos, just like stencil itself.
This came up in #131836 (and probably led to asset essential brushes all
having this wrong -- which in turn will not draw stencil masks for them
in the viewport).

NOTE: without those defaults, resetting the brush would also have this
issue.

For further steps to actually fix fully, please refer to #131836.

Pull Request: https://projects.blender.org/blender/blender/pulls/131848

source/blender/makesdna/DNA_brush_defaults.h

@@ -87,6 +87,8 @@
  \
     .stencil_pos = {256, 256}, \
     .stencil_dimension = {256, 256}, \
+    .mask_stencil_pos = {256, 256}, \
+    .mask_stencil_dimension = {256, 256}, \
  \
     /* sculpting defaults to the draw brush for new brushes */ \
     .sculpt_brush_type = SCULPT_BRUSH_TYPE_DRAW, \