[magick-users] Optimized GetPixelOpacity()

[Seth Price]

I profiled some code that I've been running quite a bit, and noticed  
that it was spending most of its time in IM line drawing routines,  
specifically GetPixelOpacity(). So, I finally took it upon myself to  
get the code running a bit faster. After two days of work, I have  
30-40% speedup, which would have saved me a month of time on the  
cluster (and will save me a month soon). Patch:
http://pricepages.org/magick/opti_magick.diff

Speedup will be most pronounced when you are drawing long lines.

The slowest part of the code was fetching the bounds in the beginning  
of each loop in the if statements like "p->bounds.y1;". Prefetching  
the data didn't seem to work, so I ended up with one main loop which  
also computed the 'winding number', so it only needed to be fetched  
in one pass, instead of two.

The rest of the stuff that I did was basic, like computing invariants  
outside of the loop(s). Cast values only once. Avoid following  
pointers. Reorder instructions to take pipelining into account, etc.

I tried to keep things readable, and not lengthen code. I wasn't sure  
exactly _what_ was being computed, so I kept the logick of the code  
identical. The only place the output was altered didn't make sense,  
and I believe it's the optimizer doing funky things. While the output  
does change, I visually don't see a difference. I put a comment  
there, so you can see how it acts on your system.

Thoughts? Does it pass your test(s)?
~Seth