avcodec: split generic YV12/NV12 copy out of SSE2 functions

This is required by the next commit. This also avoids making two copies
on non-x86 platforms, where USWC optimizations are irrelevant.
(cherry picked from commit 07d8e576c7a0dcc7863d6853f620d5a99ebba55f)

Conflicts:
	modules/codec/avcodec/copy.c

modules/codec/avcodec/copy.h

@@ -25,8 +25,10 @@
 #define _VLC_AVCODEC_COPY_H 1
 
 typedef struct {
+# ifdef CAN_COMPILE_SSE2
     uint8_t *buffer;
     size_t  size;
+# endif
 } copy_cache_t;
 
 int  CopyInitCache(copy_cache_t *cache, unsigned width);

modules/codec/avcodec/vaapi.c

@@ -310,7 +310,8 @@ static int CreateSurfaces( vlc_va_vaapi_t *p_va, void **pp_hw_ctx, vlc_fourcc_t
         goto error;
     *pi_chroma = i_chroma;
 
-    CopyInitCache( &p_va->image_cache, i_width );
+    if( unlikely(CopyInitCache( &p_va->image_cache, i_width )) )
+        goto error;
 
     /* Setup the ffmpeg hardware context */
     *pp_hw_ctx = &p_va->hw_ctx;
@@ -358,9 +359,6 @@ static int Extract( vlc_va_t *p_external, picture_t *p_picture, AVFrame *p_ff )
 {
     vlc_va_vaapi_t *p_va = vlc_va_vaapi_Get(p_external);
 
-    if( !p_va->image_cache.buffer )
-        return VLC_EGENERIC;
-
     VASurfaceID i_surface_id = (VASurfaceID)(uintptr_t)p_ff->data[3];
 
 #if VA_CHECK_VERSION(0,31,0)