Audio

add libasound2-dev travis dependency

Use travis-ci

more performance tweaks

Because that's expensive, even in the case where the conversion
is trivial. Use type assertion first. Reduces runtime cost of
imdraw.Push from ~15.3% to 8.4%, so not-quite-50% of runtime
cost of pushing points.

If you were setting imd.Color to Color objects that aren't RGBA
every single point, not much help. But if you set it and then
draw a bunch of points, this will be a big win.

Soooo. It turns out that the bunch of smallish (~4-5% of runtime)
loads associated with Len(), Unit(), Rotated(), and so on... Were
actually more like 15% or more of computational effort. I first
figured this out by creating:

	func (u Vec) Normal(v Vec) Vec

which gives you a vector normal to u->v. That consumed a lot
of CPU time, and was followed by .Unit().Scaled(imd.thickness / 2),
which consumed a bit more CPU time.

After some poking, and in the interests of avoiding UI cruft,
the final selection is
	func (u Vec) Normal() Vec

This returns the vector rotated 90 degrees, which turns out to
be the most common problem.

We never actually need the "normal" value; it's an extra calculation
we didn't need, because ijNormal is the same value early on. It's
totally possible that we could further simplify this; there's a lot
of time going into the normal computations.

updateData()'s loops checking gt.Len() turns out to have been costing
significant computation, not least because each call then in turn
called gt.vs.Stride().

revised performance tuning pull request

The computation including a call to Stride() can't be optimized away
safely because the compiler can't tell that Stride() is effectively
constant, but we know it won't change so we can make a slice pointing
at that part of the array.

CPU time for updateData goes from 26.35% to 18.65% in my test case.

A slice of points means copying every point into the slice, then
copying every point's data from the slice to TrianglesData. An
array of indicies lets the compiler make better choices.