s/c-buffer spans calculation

- transform triangle into screen space
- now sort vertices to get the most upper one.
- do interpolation from the most upper to those 2 at bottom
i.e. having [x1,y1] at top, and [x2,y2] [x3,y3] at bottom:
ix1(y) = x1 + ((x2-x1)/(y2-y1)) * (y-y1) // this is formula to compute iterpolated x coordinate from point 1 to point 2 at line y
(x2-x1)/(y2-y1) does represent slope of line
if you are proceeding from line y1 to y2 step by step,
you don''t have to do this multiplication, but you may do

ix1 = x1; slope = (x2-x1)/(y2-y1);
for ( y = y1; y <= y2; y++ ) {
ix1 += slope;
}

the same thing with line from point 1 to point 3 ...
You probably noticed, that this is very similar to line drawing... indeed it is.

Then when you reach either y2(y3) line, you will have to change the first(second) line to line from point 2(3) to point 3(2) ...

Also there are some special case, when y1==y2, or y2==y3, etc... get paper and draw few triangles to make it clear for yourself...

Have a fun. (I actually never liked this stuff, but got trough it)

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Ped - Peter Helcmanovsky - 7 Gods demo group
http://7gods.rulez.sk
First Sight Entertainment - http://members.xoom.com/fseteam
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