buffered canvas layers

[seansfkelley]

Rendering to an off-screen canvas OR stashing a rendered canvas layer to use in between expensive full renders (scaled/translated as appropriate) could improve perceived performance considerably.

[seansfkelley]

ae4520a70153765ace918a6b1527017321184b83 addresses one aspect of this issue. This ticket also applies to pan/zoom transformations: when the X or Y domains change, the layer could immediately render a translated/zoomed version of the current canvas while a throttled full render is queued up. This behavior should be opt-in for "performance mode" though, as it could make low-scale charts look uglier than necessary.