The initializer is surprisingly expensive.

Removing the call to Alpha(1) and replacing it with an inline definition
produces measurable improvements. Replacing each instance of ZV with
Vec{} further improves things. We keep an inline RGBA because there
are circumstances (mostly when using pictures) where we don't want to
have to set colors to get default behavior.

For a fairly triangle-heavy thing, this reduces time spent in SetLen
from something over 10% of execution time to around 2.5% of execution
time.