Explore replacing pad <use> tags with just the shapes

[mcous]

There's anecdotal evidence that <use> tags slow things down and that nesting them is bad. For every single "pad" shape, gerber-to-svg puts the shape in <defs>, and for every "flash", it puts a <use> in the main layer (that's a lot of uses). Then, when we start combining gerber-to-svg layers together with pcb-stackup, we end up with a bunch of nested <use>s, because the layers get put into defs and then <use>d themselves.

I'd like to compare a build that just inserts the shapes directly into the main to the current version to see if things feel faster (especially with some of the more complicated boards in the pcb-stackup integration tests).

[mcous]

Now tracking in tracespace/tracespace#80