avcodec: add Newtek SpeedHQ decoder

avcodec: add Newtek SpeedHQ decoder

This decoder can decode all existing SpeedHQ formats (SHQ0–5, 7, and 9),
including correct decoding of the alpha channel.

1080p is decoded in 142 fps on one core of my i7-4600U (2.1 GHz Haswell),
about evenly split between bitstream reader and IDCT. There is currently
no attempt at slice or frame threading, even though the format trivially
supports both.

NewTek very helpfully provided a full set of SHQ samples, as well as
source code for an SHQ2 encoder (not included) and assistance with
understanding some details of the format.