Allow absolute values when setting speed, volume and seek

[bklang]

The media output spec allows for altering playback speed and volume, as well as seeking within the stream. However all of these controls only accept offsets from the current speed, volume and location, respectively. The spec also does not define what the relative units are.

Since deltas are relatively easy to calculate, and using them introduces a possible race condition, I would prefer to be able to set each of these with absolute values as necessary.

Examples:

Speed: 1.0 should be "normal" playback speed, with floating point values of 0.5 indicating 1/2 speed and 1.5 indicating 150% playback speeds.
Volume: 77% (perhaps represented as a floating point value between 0.00 and 1.00)
Seek: may be done in samples or as a percentage of duration