vf_colorspace: Add BT-names for gamma22/28 transfer option

Allows to use values returned from API and from ffprobe directly.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavfilter/vf_colorspace.c

@@ -1055,7 +1055,9 @@ static const AVOption colorspace_options[] = {
       OFFSET(user_trc),   AV_OPT_TYPE_INT, { .i64 = AVCOL_TRC_UNSPECIFIED },
       AVCOL_TRC_RESERVED0, AVCOL_TRC_NB - 1, FLAGS, "trc" },
     ENUM("bt709",        AVCOL_TRC_BT709,        "trc"),
+    ENUM("bt470m",       AVCOL_TRC_GAMMA22,      "trc"),
     ENUM("gamma22",      AVCOL_TRC_GAMMA22,      "trc"),
+    ENUM("bt470bg",      AVCOL_TRC_GAMMA28,      "trc"),
     ENUM("gamma28",      AVCOL_TRC_GAMMA28,      "trc"),
     ENUM("smpte170m",    AVCOL_TRC_SMPTE170M,    "trc"),
     ENUM("smpte240m",    AVCOL_TRC_SMPTE240M,    "trc"),