Draw tree branches with single stroke.
Vary tree heights.
Unify lower- and upper-axis triangle shading code.
Add 8-ushort simd type.

This will support vectorising the triangle shader. It would be nice if SSE3 had
a similar instruction for 4-uint, but no such instruction exists, even in later
SSE versions.

Also, inline assembly. Yay.
Add 16-bit-low multiply simd functions.

Also reduce the base for usimd8s fractions to 65535, so that we can multiply by
the denominator and not overflow to zero.
Vectorise "z" interpolation of tscan.

Breaks hull and tiled-texture, but neither of those are being used right now,
except by the flower-pot demo.
Reduce detail level of trees.
Separate tree prop layers into two.
Make trees bigger, reduce tree count.
Increase fast brush splotch size, add fuzzing.
Increase maximum object size drawn by turtle.
Tweak tree colours.
Draw tree leaves even at level 0, and then at twice the size.
Properly include initial splotch in fast brush.
Always flush brush when drawing trees.

Makes new branches start on a graceful splotch, reduces "running" effect.
Tweak parms on shared fast brush.
Merge branch 'master' into gl-rendering

Conflicts:
	src/render/tree-props.c
Fix some issues with glbrush.

- Noise texture now has correct distribution.

- Don't discard polygons bigger than the screen.
Tweak usage of glbrush for drawing tree branches.
Refine operation of glbrush.

Now associated with a heavy-weight object with fixed colours and
parameters. This means we can move most of the data we were passing into the
uniform / texture, thus substantially reducing the bandwidth to GL.