vulkan/ffv1: fix sync issue in cached bitstream reader/writer

The issue is that there is an explicit lack of synchronization as only the very
first invocation writes symbols and updates the state, which other invocations
then store.

libavcodec/vulkan/ffv1_dec.comp

@@ -110,6 +110,8 @@ void decode_line(inout SliceContext sc, ivec2 sp, int w,
 
 #ifdef CACHED_SYMBOL_READER
         }
+
+        barrier();
         sb.v = state[gl_LocalInvocationID.x];
 #endif
     }

libavcodec/vulkan/ffv1_enc.comp

@@ -107,6 +107,7 @@ void encode_line(inout SliceContext sc, readonly uimage2D img, uint state_off,
             put_symbol(sc.c, context_off, d[1]);
 
 #ifdef CACHED_SYMBOL_READER
+        barrier();
         sb.v = state[gl_LocalInvocationID.x];
 #endif
     }