avcodec/refstruct: Don't use STRIDE_ALIGN for alignment

It was always intended that the buffers returned by
RefStruct shall have the same alignment guarantees
as the buffers returned by av_malloc(); said alignment
depends upon the arch and the enabled instruction set
and the code used STRIDE_ALIGN as a proxy for this.

Yet since 7945d30e91
there is a better way to get av_malloc's alignment:
ALIGN_64 in mem_internal.h. So use this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/refstruct.c

@@ -20,13 +20,13 @@
 #include <stdint.h>
 #include <string.h>
 
-#include "internal.h"
 #include "refstruct.h"
 
 #include "libavutil/avassert.h"
 #include "libavutil/error.h"
 #include "libavutil/macros.h"
 #include "libavutil/mem.h"
+#include "libavutil/mem_internal.h"
 #include "libavutil/thread.h"
 
 #ifndef REFSTRUCT_CHECKED
@@ -46,9 +46,9 @@
                                MKTAG('R', 'e', 'f', 'S') | (uint64_t)MKTAG('t', 'r', 'u', 'c') << 32)
 
 #if __STDC_VERSION__ >= 201112L && !defined(_MSC_VER)
-#define REFCOUNT_OFFSET FFALIGN(sizeof(RefCount), FFMAX3(STRIDE_ALIGN, 16, _Alignof(max_align_t)))
+#define REFCOUNT_OFFSET FFALIGN(sizeof(RefCount), FFMAX(ALIGN_64, _Alignof(max_align_t)))
 #else
-#define REFCOUNT_OFFSET FFALIGN(sizeof(RefCount), FFMAX(STRIDE_ALIGN, 16))
+#define REFCOUNT_OFFSET FFALIGN(sizeof(RefCount), ALIGN_64)
 #endif
 
 typedef struct RefCount {