attempt to fix unused var warnings in stb_image 2.09

stb_image.h

@@ -1,4 +1,4 @@
-/* stb_image - v2.09 - public domain image loader - http://nothings.org/stb_image.h
+/* stb_image - v2.10 - public domain image loader - http://nothings.org/stb_image.h
                                      no warranty implied; use at your own risk
 
    Do this:
@@ -146,6 +146,7 @@
 
 
    Latest revision history:
+      2.10  (2016-01-22) avoid warning introduced in 2.09
       2.09  (2016-01-16) 16-bit TGA; comments in PNM files; STBI_REALLOC_SIZED
       2.08  (2015-09-13) fix to 2.07 cleanup, reading RGB PSD as RGBA
       2.07  (2015-09-13) partial animated GIF support
@@ -3621,6 +3622,7 @@ static int stbi__zexpand(stbi__zbuf *z, char *zout, int n)  // need to make room
    while (cur + n > limit)
       limit *= 2;
    q = (char *) STBI_REALLOC_SIZED(z->zout_start, old_limit, limit);
+   STBI_NOTUSED(old_limit);
    if (q == NULL) return stbi__err("outofmem", "Out of memory");
    z->zout_start = q;
    z->zout       = q + cur;
@@ -4410,6 +4412,7 @@ static int stbi__parse_png_file(stbi__png *z, int scan, int req_comp)
                if (idata_limit == 0) idata_limit = c.length > 4096 ? c.length : 4096;
                while (ioff + c.length > idata_limit)
                   idata_limit *= 2;
+               STBI_NOTUSED(idata_limit_old);
                p = (stbi_uc *) STBI_REALLOC_SIZED(z->idata, idata_limit_old, idata_limit); if (p == NULL) return stbi__err("outofmem", "Out of memory");
                z->idata = p;
             }
@@ -6456,6 +6459,7 @@ STBIDEF int stbi_info_from_callbacks(stbi_io_callbacks const *c, void *user, int
 
 /*
    revision history:
+      2.10  (2016-01-22) avoid warning introduced in 2.09 by STBI_REALLOC_SIZED
       2.09  (2016-01-16) allow comments in PNM files
                          16-bit-per-pixel TGA (not bit-per-component)
                          info() for TGA could break due to .hdr handling