[Fluxus] motion blur fix

[RPD]

Hello, again. I believe I got to the bottom of the blur issue, and fixed
it. This patch adds a function (blur-blend-mode) which is basically identical to (blend-mode) except it applies only to the plane which is
responsible for covering the screen with your clear-colour. I also found
out that when you use blur and switch to a fisheye view with e.g.
(clip 0.01 10000), that plane appears in the middle of the screen and is the only part of the scene getting cleared. I tried making the plane
obscenely large but I could always set the near clipping plane to
something obscenely small and still not be clearing the whole screen. Also
if I put the near plane at something like 14 for a far zoom, there would be
no clearing whatsoever since that blurring plane was behind the camera. I
played with that for a while, and found that putting the near clip plane
and the blur plane together would make it always work, at least, It Works
For Me(TM). So I made it automatically do that when you run (clip).

Now you can still have fun side effects by choosing the mode you like, and
the editor visibility doesn't affect anything accidentally. I'm slowly
learning to use git, which is really why this took forever... and I really
hope it doesn't break something else like fog... GL HF :)

--Rob