Prefer zero-copy formats for non-hwaccel hardware decoders

SpacemiT K1's BSP has a version of FFmpeg that contains both
v4l2m2m and their own *_stcodec decoders. Not only is their
V4L2 driver for their mvx decoder seemingly totally broken
(output buffers are all zeros), but it doesn't have any of
the DRM_PRIME support work that is maintained out of tree.

The result is we use a broken copyback decoder instead of
a (mostly) working zero-copy decoder because we didn't
include any logic to rank non-hwaccel decoders by pixfmt
like we have with device type for hwaccel decoders.

app/streaming/video/ffmpeg.h

@@ -48,6 +48,9 @@ private:
     static
     bool isDecoderIgnored(const AVCodec* decoder);
 
+    static
+    bool isZeroCopyFormat(AVPixelFormat format);
+
     static
     int getAVCodecCapabilities(const AVCodec *codec);
 
@@ -55,6 +58,10 @@ private:
                                      int pass,
                                      QSet<const AVCodec*>& terminallyFailedHardwareDecoders);
 
+    bool tryInitializeNonHwAccelDecoder(PDECODER_PARAMETERS params,
+                                        bool requireZeroCopyFormat,
+                                        QSet<const AVCodec*>& terminallyFailedHardwareDecoders);
+
     bool tryInitializeRendererForUnknownDecoder(const AVCodec* decoder,
                                                 PDECODER_PARAMETERS params,
                                                 bool tryHwAccel);