Make IndexedTrisShader allow adding more vertex attributes.

Comments had said you could so long as you only extended, but
it wasn't ACTUALLY possible since there were important bits tied
up in private constants. It could use a little more love to make
things friendlier, but for the moment this lets you actually
get it done.

Next up? Similar-in-spirit work on QuadShader, since that's more
performant and I'd really like to use a QuadShader derivative if
at all possible.