avutil/pixdesc: add AV_PIX_FMT_FLAG_XYZ

There are already several places in the codebase that match desc->name
against "xyz", and many downstream clients replicate this behavior.
I have no idea why this is not just a flag.

Motivated by my desire to add yet another check for XYZ to the codebase,
and I'd rather not keep copy/pasting a string comparison hack.

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2023-02-09
 
 API changes, most recent first:
 
+2023-10-31 - xxxxxxxxxx - lavu 58.31.100 - pixdesc.h
+  Add AV_PIX_FMT_FLAG_XYZ.
+
 -------- 8< --------- FFmpeg 6.1 was cut here -------- 8< ---------
 
 2023-10-27 - xxxxxxxxxx - lavu 58.28.100 - channel_layout.h

libavutil/pixdesc.c

@@ -1977,8 +1977,8 @@ static const AVPixFmtDescriptor av_pix_fmt_descriptors[AV_PIX_FMT_NB] = {
             { 0, 6, 0, 4, 12 },       /* X */
             { 0, 6, 2, 4, 12 },       /* Y */
             { 0, 6, 4, 4, 12 },       /* Z */
-      },
+        },
-      /*.flags = -- not used*/
+        .flags = AV_PIX_FMT_FLAG_XYZ,
     },
     [AV_PIX_FMT_XYZ12BE] = {
         .name = "xyz12be",
@@ -1990,7 +1990,7 @@ static const AVPixFmtDescriptor av_pix_fmt_descriptors[AV_PIX_FMT_NB] = {
             { 0, 6, 2, 4, 12 },       /* Y */
             { 0, 6, 4, 4, 12 },       /* Z */
        },
-        .flags = AV_PIX_FMT_FLAG_BE,
+        .flags = AV_PIX_FMT_FLAG_XYZ | AV_PIX_FMT_FLAG_BE,
     },
 
 #define BAYER8_DESC_COMMON \

libavutil/pixdesc.h

@@ -157,6 +157,11 @@ typedef struct AVPixFmtDescriptor {
  */
 #define AV_PIX_FMT_FLAG_FLOAT        (1 << 9)
 
+/**
+ * The pixel format contains XYZ-like data (as opposed to YUV/RGB/grayscale).
+ */
+#define AV_PIX_FMT_FLAG_XYZ          (1 << 10)
+
 /**
  * Return the number of bits per pixel used by the pixel format
  * described by pixdesc. Note that this is not the same as the number

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  58
-#define LIBAVUTIL_VERSION_MINOR  30
+#define LIBAVUTIL_VERSION_MINOR  31
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \