not sure that we can call NotifyResetDone immediately.

AVDA notifies the pending bitstream buffers, then flushes the codec in case there are pictures that will arrive later.  I think that we need to do both.  Maybe dx11 has a way to clear the codec state that we could use.

at least, i think that we need to notify pending buffers.  we might also need to verify that calling Reset() on |decoder_| prevents any in-flight pictures from arriving -- they shouldn't, or we should wait for them before signalling NotifyResetDone().  i think either is okay.