FFmpegUtilities: replace variable definition with statement expression

It is needed to avoid multiple definitions of AV_GET_CODEC_CONTEXT,
which is considered as an error with '-fno-common' which is default
since gcc-10.

Fixes: #511

include/FFmpegUtilities.h

@@ -163,11 +163,10 @@
 		#define AV_FREE_CONTEXT(av_context) avcodec_free_context(&av_context)
 		#define AV_GET_CODEC_TYPE(av_stream) av_stream->codecpar->codec_type
 		#define AV_FIND_DECODER_CODEC_ID(av_stream) av_stream->codecpar->codec_id
-		auto AV_GET_CODEC_CONTEXT = [](AVStream* av_stream, AVCodec* av_codec) { \
-			AVCodecContext *context = avcodec_alloc_context3(av_codec); \
-			avcodec_parameters_to_context(context, av_stream->codecpar); \
-			return context; \
-		};
+		#define AV_GET_CODEC_CONTEXT(av_stream, av_codec) \
+			({ AVCodecContext *context = avcodec_alloc_context3(av_codec); \
+			   avcodec_parameters_to_context(context, av_stream->codecpar); \
+			   context; })
 		#define AV_GET_CODEC_PAR_CONTEXT(av_stream, av_codec) av_codec;
 		#define AV_GET_CODEC_FROM_STREAM(av_stream,codec_in)
 		#define AV_GET_CODEC_ATTRIBUTES(av_stream, av_context) av_stream->codecpar
@@ -199,11 +198,10 @@
 		#define AV_FREE_CONTEXT(av_context) avcodec_free_context(&av_context)
 		#define AV_GET_CODEC_TYPE(av_stream) av_stream->codecpar->codec_type
 		#define AV_FIND_DECODER_CODEC_ID(av_stream) av_stream->codecpar->codec_id
-		auto AV_GET_CODEC_CONTEXT = [](AVStream* av_stream, AVCodec* av_codec) { \
-			AVCodecContext *context = avcodec_alloc_context3(av_codec); \
-			avcodec_parameters_to_context(context, av_stream->codecpar); \
-			return context; \
-		};
+		#define AV_GET_CODEC_CONTEXT(av_stream, av_codec) \
+			({ AVCodecContext *context = avcodec_alloc_context3(av_codec); \
+			   avcodec_parameters_to_context(context, av_stream->codecpar); \
+			   context; })
 		#define AV_GET_CODEC_PAR_CONTEXT(av_stream, av_codec) av_codec;
 		#define AV_GET_CODEC_FROM_STREAM(av_stream,codec_in)
 		#define AV_GET_CODEC_ATTRIBUTES(av_stream, av_context) av_stream->codecpar