WRaster: Add optional support to JPEG XL

Detect if libjxl is intalled, and build-in support in raster lib.
Feature can be disabled/enabled at configure time.

wrlib/imgformat.h

@@ -38,12 +38,13 @@ typedef enum {
 	IM_PPM     =  4,
 	IM_JPEG    =  5,
 	IM_GIF     =  6,
-	IM_WEBP    =  7
+	IM_WEBP    =  7,
+	IM_JXL     =  8
 } WRImgFormat;
 
 /* How many image types we have. */
 /* Increase this when adding new image types! */
-#define IM_TYPES    7
+#define IM_TYPES    8
 
 /*
  * Function for Loading in a specific format
@@ -64,6 +65,10 @@ RImage *RLoadPNG(RContext *context, const char *file);
 RImage *RLoadJPEG(const char *file);
 #endif
 
+#ifdef USE_JXL
+RImage *RLoadJXL(const char *file);
+#endif
+
 #ifdef USE_GIF
 RImage *RLoadGIF(const char *file, int index);
 #endif