Web audio api for SDL library.

Builds on unmerged pull request #971 and implements Web Audio API support to SDL. Firefox continues to use Mozilla Audio Data API because 1) Web Audio API is not yet supported in latest Firefox stable and 2) it is superior in terms of audio quality due to the ability to perform back-to-back stitching of audio data.

Also fixes several queueing issues with audio synthesization, and adds a browser test to check the support of different SDL audio formats.

HTML5 ScummVM tested to work great in both Firefox and Chrome with the new audio code.