qsvenc: Add an option to disable MFE mode

Provide proper aliases to enable/disable MFE.

The numeric values are ambiguous and misleading (e.g: user may misunderstand
setting mfmode to 1 is to enable MFE but actually it is to disable MFE, and
set it to be 5 or above is meaningless).

MFX_MF_MANUAL hasn't been exposed since it is to be implemented.
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/qsvenc_h264.c

@@ -94,7 +94,9 @@ static const AVOption options[] = {
     { "aud", "Insert the Access Unit Delimiter NAL", OFFSET(qsv.aud), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, 1, VE},
 
 #if QSV_HAVE_MF
-    { "mfmode", "Multi-Frame Mode", OFFSET(qsv.mfmode), AV_OPT_TYPE_INT, { .i64 = MFX_MF_AUTO }, 0, INT_MAX, VE },
+    { "mfmode", "Multi-Frame Mode", OFFSET(qsv.mfmode), AV_OPT_TYPE_INT, { .i64 = MFX_MF_AUTO }, MFX_MF_DEFAULT, MFX_MF_AUTO, VE, "mfmode"},
+    { "off"    , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_MF_DISABLED }, INT_MIN, INT_MAX,     VE, "mfmode" },
+    { "auto"   , NULL, 0, AV_OPT_TYPE_CONST, { .i64 = MFX_MF_AUTO     }, INT_MIN, INT_MAX,     VE, "mfmode" },
 #endif
 
     { NULL },