Efficient support for preload='metadata'

[bvibber]

Currently only preload='none' and 'auto' are supported; if 'metadata' is specified we'll still load a one-megabyte chunk and fire up the decoders.

Once network layer is redone should be easier to have read only a short bit at beginning. May be able to avoid starting the decoder threads depending on whether it's supposed to be exposing file width/height or just duration. (check spec)