don't make spurious calls to GetFrame (waste of CPU)
- fix DecodeFrame setting FRAME_DECODED without actually decoding a frame on EOF
after loop
update comment
(probably not related to "movie not looping" problems)
We could make MovieDecoder the arch class, with MovieTexture_Generic
replacing RageMovieTexture, but it forces a pull-based API, and DShow
insists on being callback-based ...