Draw oblique slices of the data

Adds drawing oblique slices of the volume to be visualized.

Calculates the slice texture coordinates by performing ray-plane intersection tests of the cube and plane intersecting it. Rotates the texture coordinates into the XY-plane to find corresponding vertex coordinates used to place the slice on screen. Calculates a convex hull of the vertices to order them in an order suitable for drawing as TRIANGLE_FAN.