Implement @elemoine's requestRenderFrame architecture
Port WebGL renderer to requestRenderFrame
Pass time to render functions and rename them to renderFrame_
Add some WebGL renderer FIXMEs
Use requestRenderFrame API in interactions
Only appending tiles to the dom after load

A port of f46cdb4 and 4242ef40 from animation-frame branch.
Demonstrating user-controlled animation

Inspired from 840ffb6
Port DOM renderer to request render frame architecture
Clarify comment
Merge pull request #92 from twpayne/request-render-frame

Request render frame