Move updateRenderables to clean API

[kkaefer]

the updateRenderables algorithm currently expects specific maps and collections; We should instead hide this behind an API to simplify testing and expansion.

[kkaefer]

This converts the updateRenderables algorithm to take a few lambdas that perform the respective actions of obtaining a TileData object, creating, retaining and rendering it. In the test suite, we can inject a log recorder into these to verify that the actions are executed in the correct order.

Prerequisite for #123

[kkaefer]

/cc @jfirebaugh

[jfirebaugh]

Didn't go too deep on this but it seems like a logical path to making it more testable. Should we rename update_renderables_impl.hpp to update_renderables.hpp, since everything includes the impl.hpp anyway?