transparency.
The issue is that we are rendering to a 0..1 clamped sRGB buffer with
unpremultiplied alpha, where the correct thing to do would be to render
to an unclamped linear premultiplied alpha buffer. Then we would just
make fire purely emissive without affecting the alpha channel at all,
but that doesn't work here.
So for now, draw fire and smoke separately using different shaders and
blend modes, like it used to before the smoke programs were rewritten
(see rB0372b642).
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
