Remove the lag between layers.

[manthey]

Pedantically, this wasn't a lag, but rather missed frames.

A variety of actions can trigger rerendering. We want to perform animations before other updates. To ensure that renderings are well ordered and not called too often, we were removing an animation frame request and then creating a new one to ensure that the callback happened at the end of the animation frame cycle. However, sometimes this would defer the animation frame callback until a later cycle rather than just at the end of the next cycle.

By using a single animation queue, we ensure that get all of the callbacks we want in the animation cycle. This has the fringe benefit that it has less overhead in Chrome. Furthermore, by using a shared array for the animation queue, two map instances can be synchronized, whereas, before, there was likely to be a lag between them.

A few other minor optimizations have been made: vgl textures are built in the _build step not in the animation frame. The vgl camera is only updated just as needed, so if two pan events happen in a single frame, only one update will occur.

Resolves issue #526.

[codecov-io]

Current coverage is 86.90% (diff: 94.11%)

Merging #644 into master will decrease coverage by <.01%

@@             master       #644   diff @@
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  Files            87         87          
  Lines          8571       8585    +14   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           7449       7461    +12   
- Misses         1122       1124     +2   
  Partials          0          0          

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[aashish24]

looks reasonable to me; I thought we merged this already but I didn't +1 it.