avcodec/decode: Reset MMX state for receive_frame decoders, too

FFmpeg's assembly code currently does not abide by the
plattform-specific ABIs wrt its handling of the X86 MMX flag:
Resetting the MMX state is deferred to avoid doing it multiple times
instead of ensuring that the CPU is in floating point state
upon return from any function.

Furthermore, resetting said state is sometimes done generically,
namely for all the decoders using the ordinary decode callback;
yet this is not done for the decoders using the receive_frame API.

This led to problems when MJPEG (and the MJPEG-based decoders)
were switched to the receive_frame API in commit
e9a2a87773, because ff_mjpeg_decode_sos()
only resets the MMX state on success, not on failure.
Such issues are probably still possible with SMVJPEG, which still
uses the receive_frame API. See issue #10210.

This commit therefore also resets the MMX state for
the receive_frame API to avoid any more surprises of this sort.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/decode.c

@@ -547,6 +547,7 @@ static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
 
     if (codec->cb_type == FF_CODEC_CB_TYPE_RECEIVE_FRAME) {
         ret = codec->cb.receive_frame(avctx, frame);
+        emms_c();
     } else
         ret = decode_simple_receive_frame(avctx, frame);