Mark GRAY8 format as pseudo-paletted.

This fixes that the GIF encoder crashes with it because
it has no palette.
And the arguments for the pseudopalette apply to gray8 as
much as to RGB8 etc.
In addition the changes required in lavfi should be needed anyway
when adding support for RGB8 etc.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavfilter/drawutils.c

@@ -134,7 +134,7 @@ int ff_draw_init(FFDrawContext *draw, enum PixelFormat format, unsigned flags)
 
     if (!desc->name)
         return AVERROR(EINVAL);
-    if (desc->flags & ~(PIX_FMT_PLANAR | PIX_FMT_RGB))
+    if (desc->flags & ~(PIX_FMT_PLANAR | PIX_FMT_RGB | PIX_FMT_PSEUDOPAL))
         return AVERROR(ENOSYS);
     for (i = 0; i < desc->nb_components; i++) {
         c = &desc->comp[i];

libavfilter/vf_pad.c

@@ -262,7 +262,7 @@ static void start_frame(AVFilterLink *inlink, AVFilterBufferRef *inpicref)
     AVFilterBufferRef *outpicref = avfilter_ref_buffer(inpicref, ~0);
     int plane;
 
-    for (plane = 0; plane < 4 && outpicref->data[plane]; plane++) {
+    for (plane = 0; plane < 4 && outpicref->data[plane] && pad->draw.pixelstep[plane]; plane++) {
         int hsub = pad->draw.hsub[plane];
         int vsub = pad->draw.vsub[plane];
 

libavutil/pixdesc.c

@@ -220,6 +220,7 @@ const AVPixFmtDescriptor av_pix_fmt_descriptors[PIX_FMT_NB] = {
         .comp = {
             { 0, 0, 1, 0, 7 },        /* Y */
         },
+        .flags = PIX_FMT_PSEUDOPAL,
     },
     [PIX_FMT_MONOWHITE] = {
         .name = "monow",