Bring the open2() call back into avcodec

I only need to pass the format to the custom get_format
function, not this* which is even wrong if user moves
the Decoder object.

src/ffmpeg/avcodec.cpp

@@ -6,6 +6,25 @@ extern "C" {
 #include <string>
 
 namespace mgs::ffmpeg {
+namespace {
+::AVPixelFormat get_favourite_format_ifp (::AVCodecContext* s, const ::AVPixelFormat* fmt) {
+	::AVPixelFormat fav_format;
+	std::copy(
+		reinterpret_cast<const char*>(&s->opaque),
+		reinterpret_cast<const char*>(&s->opaque) + sizeof(::AVPixelFormat),
+		reinterpret_cast<char*>(&fav_format)
+	);
+
+	const ::AVPixelFormat* curr_fmt = fmt;
+	while (*curr_fmt != -1) {
+		if (*curr_fmt == fav_format)
+			return fav_format;
+		++curr_fmt;
+	}
+	return ::avcodec_default_get_format(s, fmt);
+}
+} //unnamed namespace
+
 AVCodec::AVCodec (::AVCodecID id) :
 	m_avcodec(::avcodec_find_decoder(id))
 {
@@ -27,7 +46,17 @@ Decoder AVCodec::make_decoder (unsigned int width, unsigned int height, ::AVPixe
 
 	context->width = width;
 	context->height = height;
-	context->codec = codec;
+	context->get_format = &get_favourite_format_ifp;
+
+	static_assert(sizeof(void*) >= sizeof(::AVPixelFormat));
+	std::copy(
+		reinterpret_cast<const char*>(&dst_format),
+		reinterpret_cast<const char*>(&dst_format) + sizeof(::AVPixelFormat),
+		reinterpret_cast<char*>(&context->opaque)
+	);
+
+	if (::avcodec_open2(context.get(), codec, nullptr) < 0)
+		throw std::runtime_error("Could not open codec");
 
 	return {std::move(context), dst_format};
 }

src/ffmpeg/decoder.cpp

@@ -13,19 +13,6 @@ namespace mgs::ffmpeg {
 namespace {
 constexpr int Align = 32;
 
-::AVPixelFormat get_favourite_format_ifp (::AVCodecContext* s, const ::AVPixelFormat* fmt) {
-	assert(s->opaque);
-
-	const ::AVPixelFormat* curr_fmt = fmt;
-	const auto fav_format = static_cast<const Decoder*>(s->opaque)->destination_format();
-	while (*curr_fmt != -1) {
-		if (*curr_fmt == fav_format)
-			return fav_format;
-		++curr_fmt;
-	}
-	return ::avcodec_default_get_format(s, fmt);
-}
-
 } //unnamed namespace
 
 Decoder::Decoder (UniqueAVCodecContext&& context, ::AVPixelFormat dst_format) :
@@ -35,13 +22,6 @@ Decoder::Decoder (UniqueAVCodecContext&& context, ::AVPixelFormat dst_format) :
 	m_frame2(::av_frame_alloc()),
 	m_dst_format(dst_format)
 {
-	assert(not m_context->opaque);
-	assert(m_context->codec);
-	m_context->opaque = this;
-	m_context->get_format = &get_favourite_format_ifp;
-
-	if (::avcodec_open2(m_context.get(), m_context->codec, nullptr) < 0)
-		throw std::runtime_error("Could not open codec");
 }
 
 Decoder::~Decoder() noexcept = default;