Some improvements for the Cage to avoid black lines on perimeter
of the cage.

GeometryUtils: pointInTriangle()
  - QPointF p() is the position of the top-left corner of the voxel.
  - depending how the lines of the triangle are defined, it is 
    possible for the voxel to be almost contained in the triangle
    but with the top-left corner being outside.
  - now check all 4 corners of the voxel as the test for inside
    the triangle. 

TriangleWarp.h:
  - srcP.toPoint() does rounding to nearest integer (corner of voxel).
  - QPoint( std::floor(srcP.x()), std::floor(srcP.y()) rounds to
    the nearest top-left corner of voxel. 
  - this is a bit tricky as to which test to use for inside the
    bounding rectangle. 
  - maybe should round to nearest centroid of voxel?
  - the effect can be a shift/translation of 1/2 voxel that we
    see sometimes.
  - I added bilinear interpolation for the Cage but the experiment
    has been disabled.
  - floor() should be more compatible with integerMoveOnly=true.