img_format: add alias for ffmpeg pal8 format

For the next commit.

video/fmt-conversion.c

@@ -33,6 +33,7 @@ static const struct {
     {IMGFMT_ABGR, AV_PIX_FMT_ABGR},
     {IMGFMT_RGBA, AV_PIX_FMT_RGBA},
     {IMGFMT_RGB24, AV_PIX_FMT_RGB24},
+    {IMGFMT_PAL8,  AV_PIX_FMT_PAL8},
     {IMGFMT_UYVY,  AV_PIX_FMT_UYVY422},
     {IMGFMT_NV12,  AV_PIX_FMT_NV12},
 #if LIBAVUTIL_VERSION_MICRO >= 100 && LIBAVUTIL_VERSION_INT >= AV_VERSION_INT(56, 27, 0)

video/img_format.h

@@ -192,6 +192,9 @@ enum mp_imgfmt {
     // Accessed with bit-shifts, uint32_t units.
     IMGFMT_RGB30,               // 2pad 10r 10g 10b (MSG to LSB)
 
+    // AV_PIX_FMT_PAL8
+    IMGFMT_PAL8,
+
     // Hardware accelerated formats. Plane data points to special data
     // structures, instead of pixel data.
     IMGFMT_VDPAU,           // VdpVideoSurface