Add experimental support for JPEG-XL, requires libvips with libjxl

The prebuilt binaries do not include support for this format.

src/pipeline.h

@@ -182,6 +182,10 @@ struct PipelineBaton {
   int heifEffort;
   std::string heifChromaSubsampling;
   bool heifLossless;
+  double jxlDistance;
+  int jxlDecodingTier;
+  int jxlEffort;
+  bool jxlLossless;
   VipsBandFormat rawDepth;
   std::string err;
   bool withMetadata;
@@ -335,6 +339,10 @@ struct PipelineBaton {
     heifEffort(4),
     heifChromaSubsampling("4:4:4"),
     heifLossless(false),
+    jxlDistance(1.0),
+    jxlDecodingTier(0),
+    jxlEffort(7),
+    jxlLossless(false),
     rawDepth(VIPS_FORMAT_UCHAR),
     withMetadata(false),
     withMetadataOrientation(-1),