* Cache icon
* Append rather than insert
* Do fewer things on update
* Don't create a fill unless needed
* Don't apply tag and member classes (never used)
* Drop down to raw setAttribute (d3 is slow :trollface:)
At z16 and below, tagged points are rendered with a small
dot in the center. At z17 and above, they are rendered with
a maki icon.
See the test rendering page for examples.
Fixes#381.
Now that Graph#parentWays is cached, the specialized Graph#parentStructure
method is no longer necessary.
This commit also demonstrates that it's relatively easy to write specs for
rendering now!
