avcodec: allocate one additional hardware surface for MPEG-2

Required to avoid visual glitches when running out of free surfaces
with avcodec and vaapi/dxva2 backends.

Fix #10868

modules/codec/avcodec/dxva2.c

@@ -865,6 +865,10 @@ static int DxCreateVideoDecoder(vlc_va_dxva2_t *va,
     case AV_CODEC_ID_H264:
         surface_count = 16 + va->thread_count + 2;
         break;
+    case AV_CODEC_ID_MPEG1VIDEO:
+    case AV_CODEC_ID_MPEG2VIDEO:
+        surface_count = 2 + 2;
+        break;
     default:
         surface_count = 2 + 1;
         break;

modules/codec/avcodec/vaapi.c

@@ -113,7 +113,7 @@ static int Open( vlc_va_t *va, int i_codec_id, int i_thread_count )
     case AV_CODEC_ID_MPEG1VIDEO:
     case AV_CODEC_ID_MPEG2VIDEO:
         i_profile = VAProfileMPEG2Main;
-        i_surface_count = 2+1;
+        i_surface_count = 2 + 2;
         break;
     case AV_CODEC_ID_MPEG4:
         i_profile = VAProfileMPEG4AdvancedSimple;