avformat/matroskadec: Check min_luminance more thoroughly

In the absence of an explicitly coded minimal luminance, the current
code inferred it to be -1, an invalid value. Yet it did not check the
value lateron at all, so that if a valid maximum luminance is
encountered, but no minimal luminance, an invalid minimal luminance of
-1 is exported. If an minimal luminance element with a negative value is
present, it is exported, too. This can be simply fixed by adding a check
for the value of the element.

Yet given that a minimal luminance of zero Cd/m² is legal and can be
coded with a length of zero, we must not use a fake default value to
find out whether the element is present or not. Therefore this patch
uses an explicit counter for it.

While just at it, also check for max_luminance > min_luminance.

Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskadec.c

@@ -168,7 +168,7 @@ typedef struct MatroskaMasteringMeta {
     double white_x;
     double white_y;
     double max_luminance;
-    double min_luminance;
+    CountedElement min_luminance;
 } MatroskaMasteringMeta;
 
 typedef struct MatroskaTrackVideoColor {
@@ -452,7 +452,7 @@ static EbmlSyntax matroska_mastering_meta[] = {
     { MATROSKA_ID_VIDEOCOLOR_BY, EBML_FLOAT, 0, 0, offsetof(MatroskaMasteringMeta, b_y) },
     { MATROSKA_ID_VIDEOCOLOR_WHITEX, EBML_FLOAT, 0, 0, offsetof(MatroskaMasteringMeta, white_x) },
     { MATROSKA_ID_VIDEOCOLOR_WHITEY, EBML_FLOAT, 0, 0, offsetof(MatroskaMasteringMeta, white_y) },
-    { MATROSKA_ID_VIDEOCOLOR_LUMINANCEMIN, EBML_FLOAT, 0, 0, offsetof(MatroskaMasteringMeta, min_luminance), { .f=-1 } },
+    { MATROSKA_ID_VIDEOCOLOR_LUMINANCEMIN, EBML_FLOAT, 1, 0, offsetof(MatroskaMasteringMeta, min_luminance) },
     { MATROSKA_ID_VIDEOCOLOR_LUMINANCEMAX, EBML_FLOAT, 0, 0, offsetof(MatroskaMasteringMeta, max_luminance) },
     CHILD_OF(matroska_track_video_color)
 };
@@ -2110,7 +2110,10 @@ static int mkv_parse_video_color(AVStream *st, const MatroskaTrack *track) {
         mastering_meta->g_x > 0 && mastering_meta->g_y > 0 &&
         mastering_meta->b_x > 0 && mastering_meta->b_y > 0 &&
         mastering_meta->white_x > 0 && mastering_meta->white_y > 0;
-    has_mastering_luminance = mastering_meta->max_luminance > 0;
+    has_mastering_luminance = mastering_meta->max_luminance >
+                                  mastering_meta->min_luminance.el.f  &&
+                              mastering_meta->min_luminance.el.f >= 0 &&
+                              mastering_meta->min_luminance.count;
 
     if (color->matrix_coefficients != AVCOL_SPC_RESERVED)
         st->codecpar->color_space = color->matrix_coefficients;
@@ -2169,7 +2172,7 @@ static int mkv_parse_video_color(AVStream *st, const MatroskaTrack *track) {
         }
         if (has_mastering_luminance) {
             metadata->max_luminance = av_d2q(mastering_meta->max_luminance, INT_MAX);
-            metadata->min_luminance = av_d2q(mastering_meta->min_luminance, INT_MAX);
+            metadata->min_luminance = av_d2q(mastering_meta->min_luminance.el.f, INT_MAX);
             metadata->has_luminance = 1;
         }
     }