Beside the obvious ARB -> GLSL change, the texture slicing algorithm had
to be rewritten.
Although this new algorithm has the same behaviour as the old one (view
aligned slicing), it works with an arbitrary number of slices (which
could eventually be set by the user), which means we can preallocate the
buffer. The previous algorithm would slice from the begining to the end
of the volume's bbox, and draw the slices as it generates them.
Also support for ARB program was removed.
Patch by myself, with some minor fixes by Brecht.
Reviewers: brecht, #opengl_gfx
Differential Revision: https://developer.blender.org/D1694
