TODO for Phase 2 (Editor)

support massive maps
- render to a bitmap and clip that in?
- sometimes UI zooms in at once, then unzooms slowly. drop events?
different UIs
- 3D UI sharing the same structure as the 2D one
- svg export some area, for manual mockups
- web version
  - ggez, quicksilver, unrust could work
easy UI bugs
- big maps start centered over emptiness
- deal with overlapping keys that still kinda happen (sim ctrl, escape game)
  - and missing keys, like no tooltip for turns, since they're only shown in editors now
- bug: do need to recalculate current_selection whenever anything potentially changes camera, like follow
traffic signal editor
- button to reset intersection to original cycles
- turns can belong to multiple cycles; the colors become slightly meaningless
- support left turn yield
stop sign editor
- cant have no stop signs for two roads whose center line crosses
  - infer default policy
be able to change road directions
tests that edits + reload from scratch are equivalent
undo support!