avifdec: Change default value of --jobs to "all"

Analogous to https://github.com/AOMediaCodec/libavif/issues/1522.

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9525ca0..9f23bad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,7 +55,8 @@
   error instead of being ignored.
 * API calls now return AVIF_RESULT_OUT_OF_MEMORY instead of aborting on memory
   allocation failure.
-* avifenc: Change the default value of the --jobs option from 1 to "all".
+* avifdec and avifenc: Change the default value of the --jobs option from 1 to
+  "all".
 * Update avifCropRectConvertCleanApertureBox() to the revised requirements in
   ISO/IEC 23000-22:2019/Amd. 2:2021 Section 7.3.6.7.
 * AVIF files with an exif_tiff_header_offset pointing at another byte than the

apps/avifdec.c

diff --git a/apps/avifdec.c b/apps/avifdec.c
index 5e2688e..435daba 100644
--- a/apps/avifdec.c
+++ b/apps/avifdec.c
@@ -36,7 +36,7 @@
     printf("Options:\n");
     printf("    -h,--help         : Show syntax help\n");
     printf("    -V,--version      : Show the version number\n");
-    printf("    -j,--jobs J       : Number of jobs (worker threads, default: 1. Use \"all\" to use all available cores)\n");
+    printf("    -j,--jobs J       : Number of jobs (worker threads). Use \"all\" to potentially use as many cores as possible (default: all)\n");
     printf("    -c,--codec C      : AV1 codec to use (choose from versions list below)\n");
     printf("    -d,--depth D      : Output depth [8,16]. (PNG only; For y4m, depth is retained, and JPEG is always 8bpc)\n");
     printf("    -q,--quality Q    : Output quality [0-100]. (JPEG only, default: %d)\n", DEFAULT_JPEG_QUALITY);
@@ -64,7 +64,7 @@
     const char * inputFilename = NULL;
     const char * outputFilename = NULL;
     int requestedDepth = 0;
-    int jobs = 1;
+    int jobs = -1;
     int jpegQuality = DEFAULT_JPEG_QUALITY;
     int pngCompressionLevel = -1; // -1 is a sentinel to avifPNGWrite() to skip calling png_set_compression_level()
     avifCodecChoice codecChoice = AVIF_CODEC_CHOICE_AUTO;
@@ -225,6 +225,10 @@
         ++argIndex;
     }
 
+    if (jobs == -1) {
+        jobs = avifQueryCPUCount();
+    }
+
     if (!inputFilename) {
         syntax();
         return 1;