clip space:
 such that xyz [-1, 1] after division
 w [near, far]
 => [-near, far]

-|w| < x/y/z < +|w|
 x and y have two valid regions (in front and behind the camera)
 z only has one valid region

- load and transform triangle vertices
- init interpolators
- store in output buffer (6 words)

clip buffer layout
0 pos[1]
1 coef[1] 0 0 1 0
2 pos[2]
3 coef[2] 0 0 0 1
4 pos[0]
5 coef[0] 0 1 0 0

VI03	input poly pointer
VI04	input poly end/output poly start
VI05	?full clip flag
VI07	output buffer pointer
VI08	output poly pointer
VI09	final vertex count

Expand:
 poly: input buffer -> ExpandBuffer (NB: max 42 verts, 0x54 qws, bigger than buffer!)
 mesh ExpandBuffer -> ClippingBuffer
Process:
 ClippingBuffer -> ExpandBuffer


Zmin - near
-----------
VF06.xy: p0.z, p1.z
VF07.xy: p01.xy - near
VF08: p2 (next p1)
VF16: p0
VF17: c0
VF18: p1
VF19: c1
VF20.xyz: interpolated clip pos
VF20.w: p1.z-p0.z
interpolation:
	(p0.z * (p1.z-near) - p1.z * (p0.z-near)) / (p1.z - p0.z)
 =	(p0.z * (p1.z-near) + p1.z * (near-p0.z)) / (p1.z - p0.z)

Zmax - far
VF06.xy: -p0.z, -p1.z
VF07.xy: far - p01.xy
interpolation:
	(p0.z * (far-p1.z) - p1.z * (far-p0.z)) / (p0.z - p1.z)
 =	(p0.z * (far-p1.z) + p1.z * (p0.z-far)) / (p0.z - p1.z)