New buffer is created on every write

When using the Web Audio API, a new buffer is created on every write. This sucks, and means it's practically impossible to play sound than is longer than the length of the write.

Ideally, we should have a callback for filling the buffer, like the Web Audio API. As an interim solution, dynamicaudio.js should maintain its own buffer somehow.

Start by reading dynamicaudio.js, focusing on the write path and how it uses the Web Audio API and Flash shim. Determine how buffer lifetime currently relates to each write and how a persistent buffer or filling callback should behave. Done means longer audio can be played without creating a new buffer for every write, with behavior consistent across supported backends.

Written by the indexing model from the issue text.