Change gzgetc() to a macro for speed (~40% speedup in testing).

gzguts.h

@@ -84,18 +84,20 @@
 
 /* internal gzip file state data structure */
 typedef struct {
+        /* exposed contents for gzgetc() macro */
+    struct gzFile_s x;      /* "x" for exposed */
+                            /* x.have: number of bytes available at x.next */
+                            /* x.next: next output data to deliver or write */
+                            /* x.pos: current position in uncompressed data */
         /* used for both reading and writing */
     int mode;               /* see gzip modes above */
     int fd;                 /* file descriptor */
     char *path;             /* path or fd for error messages */
-    z_off64_t pos;          /* current position in uncompressed data */
     unsigned size;          /* buffer size, zero if not allocated yet */
     unsigned want;          /* requested buffer size, default is GZBUFSIZE */
     unsigned char *in;      /* input buffer */
     unsigned char *out;     /* output buffer (double-sized when reading) */
-    unsigned char *next;    /* next output data to deliver or write */
         /* just for reading */
-    unsigned have;          /* amount of output data unused at next */
     int eof;                /* true if end of input file reached */
     z_off64_t start;        /* where the gzip data started, for rewinding */
     int how;                /* 0: get header, 1: copy, 2: decompress */