Speed up cimg::fsize

Author: apocelipes

CImg.h

@@ -5795,18 +5795,31 @@ namespace cimg_library {
     **/
     inline cimg_int64 fsize(std::FILE *const file) {
       if (!file) return (cimg_int64)-1;
+#if cimg_OS==1
+      int fd = fileno(file);
+      struct stat st;
+      if (fstat(fd,&st) < 0) return (cimg_int64)-1;
+      return (cimg_int64)st.st_size;
+#else
       const cimg_long pos = std::ftell(file);
       cimg::fseek(file,0,SEEK_END);
       const cimg_int64 siz = (cimg_int64)std::ftell(file);
       cimg::fseek(file,pos,SEEK_SET);
       return siz;
+#endif
     }
 
     inline cimg_int64 fsize(const char *const filename) {
+#if cimg_OS==1
+      struct stat st;
+      if (stat(filename,&st) < 0) return (cimg_int64)-1;
+      return (cimg_int64)st.st_size;
+#else
       std::FILE *const file = cimg::std_fopen(filename,"rb");
       const cimg_int64 siz = fsize(file);
       cimg::fclose(file);
       return siz;
+#endif
     }