Default improved (faster) orthogonal volume slice drawing on. There are two...

Default improved (faster) orthogonal volume slice drawing on.  There are two algorithms for drawing volume slices.  The first method (original) draws each voxel as a quad, independent of all other voxels.  This first method is more efficient when a 'small' number of voxel in the slice (e.g.: label volume) are drawn.  The second method (new) submits all vertices (coord/normal/rgba) to OpenGL and then draws the voxels using the vertex indices of the quads.  The second method is more efficient when most voxels in the slice are drawn (e.g.: anatomical volume).  Now, when a volume slice is about to be drawn, an estimate of the number of bytes submitted to OpenGL is computed for both methods and the method that requires the least number of bytes is used.  Timings show a twenty to forty percent reduction in volume slice drawing.