Modify CCGSubsurf to subdivide an arbitrary number of (float) layers.

The layout of vert data in CCGSubSurf is almost the same; previously
it was three floats (for xyz coordinate) optionally followed by three
floats for the normal. The only change is that the first three floats
can now be any number of floats.

* _getSubSurf takes a numLayers parameter to set the number of layers,
  stored in CCGMeshIFC.numLayers.

* All calls to _getSubSurf currently have numLayers set to 3, except
  for UV subsurf, where it is reduced to 2 (with a corresponding
  change when reading the results out to use float (*)[2] rather than
  float (*)[3].)

* The various VertData* macros in CCGSubSurf.c are now functions that
  take a CCGSubSurf pointer, which provides access to CCGMeshIFC,
  which has numLayers.

* Add ccgSubSurf_setNumLayers() to the API. Only changes the number of
  layers that get subdivided, doesn't change the amount of memory
  allocated. So if space for N layers is allocated, it's safe to set
  the number of layers to less than N, but not more.

* The rest of the changes are just adding the 'ss' parameter.
Add CCGKey/CCGElem for accessing CCGSubSurf elements.

CCGKey caches information about the CCGSubSurf element layout. This
data, along with the CCG_* inline functions, allows access to
CCGSubSurf elements with an arbitrary number of layers (as opposed to
the hardcoded DMGridData structure which assumes xyz coordinates
followed by three normal components.)

The CCGElem structure is declared but not defined anywhere, just used
as a convenient type.
Replace hardcoded DMGridData structure with CCGElem/CCGKey.

* Changes to DerivedMesh interface: DMGridData has been removed,
  getGridData() now returns an array of CCGElem pointers. Also added
  getGridKey() to initialize a CCGKey (implemented only by
  CCGDerivedMesh.)

* PBVH: added BLI_pbvh_get_grid_key().

* A lot of code is affected, but mainly is just replacing
  DMGridData.co, DMGridData.no, and sizeof(DMGridData) with the
  CCG_*_elem functions, removing the reliance on grid elements of
  exactly six floats.
Add DNA and customdata entries for paint masks.

CD_PAINT_MASK is a layer of per-vertex floats for non-multires
meshes. Multires meshes use CD_GRID_PAINT_MASK, which is a layer of
per-loop GridPaintMask structures. GridPaintMask is similar to MDisp,
but contains an array of scalar floats.

Note: the GridPaintMask could be folded into MDisp, but this way
should be easier to add mask layers in the future (if we do fold
GridPaintMask into MDisp, the mask array should probably be an array
of arrays with a 'totmask' field so that mask layers can be easily
supported.)

Includes blenload read/write support for CD_PAINT_MASK and
CD_GRID_PAINT_MASK.
Add access to mesh vertex customdata to the PBVH.
Add paint mask access to the PBVH vertex iterator.
Add GridPaintMask accessor to paint.c.
Add mask support to CCGSubSurf and multires.

* Add new CCG function ccgSubSurf_setAllocMask(). Similar to to
  ccgSubSurf_setCalcVertexNormals(), it sets whether the CCG elements
  have a mask layer and what that layer's offset is. Unlike normals
  however, it doesn't change any behavior during CCG calculation; it's
  there only to give CCGKey information on the mask.

* Add a new flag to _getSubSurf(), CCG_ALLOC_MASK. If set, space for
  an extra layer is allocated, but the number of CCG layers is not set
  to include it. This is done because GridPaintMasks are absolute,
  rather than being relative to the subdivided output (as MDisp
  displacements are), so we skip subdividing paint masks here.

* Add a new flag to subsurf_make_derived_from_derived(),
  SUBSURF_ALLOC_PAINT_MASK. This controls whether CCG_ALLOC_MASK is
  set for _getSubSurf(). Related, masks are never loaded in during
  ss_sync_from_derivedmesh(). After subdivision is finished, if the
  alloc mask flag is set, the number of CCG layers is increase to 4
  with ccgSubSurf_setNumLayers().

* Add a new flag to multires_make_from_derived(),
  MULTIRES_ALLOC_PAINT_MASK. Not all multires functions need paint
  mask data (e.g. multiresModifier_base_apply.) This flag is always
  set in MOD_multires.c so that subdividing a mesh with a mask updates
  properly even when not in sculpt mode.

* Update multiresModifier_disp_run() to apply, calculate, and add mask
  elements. It's almost the same as the existing operations with xyz
  coordinates, but treats masks as absolute rather than displacements
  relative to subdivided values.

* Update multires_customdata_delete to free CD_GRID_PAINT_MASK in
  addition to CD_MDISPS.

* Update multires_del_higher() to call the new function
  multires_grid_paint_mask_downsample(), which allocates a
  lower-resolution paint mask grid and copies values over from the
  high-resolution grid.
Copy GridPaintMask to vertex paint mask when applying multires.

Adds new subsurf_copy_grid_paint_mask() function similar to
subsurf_copy_grid_hidden().
Ensure mask layers are always present in sculpt mode.
Add undo/redo support for paint masks.
Use paint mask when calculating sculpt strength.
Add new mask-brush icon from Julio Iglesias.
Add mask brush for sculpt mode.

The mask brush currently has two modes, 'draw' and 'smooth'.
Add MKEY to the sculpt keymap to toggle the mask brush.
Support shift-key smoothing of the mask.
Add a paint mask operator to clear, fill, or invert the mask.
Add support for hiding masked regions
Add keymap and menu entries for masking.
Use VertexBufferFormat for multires VBO.