stb_image, stb_image_write: Fix compare sign warnings

For the stb_image fix, also replace the magic 288 with a more
descriptive name while I'm at it.

Fixes #1100

stb_image.h

@@ -3993,6 +3993,7 @@ static int stbi__jpeg_info(stbi__context *s, int *x, int *y, int *comp)
 // fast-way is faster to check than jpeg huffman, but slow way is slower
 #define STBI__ZFAST_BITS  9 // accelerate all cases in default tables
 #define STBI__ZFAST_MASK  ((1 << STBI__ZFAST_BITS) - 1)
+#define STBI__ZNSYMS 288 // number of symbols in literal/length alphabet
 
 // zlib-style huffman encoding
 // (jpegs packs from left, zlib from right, so can't share code)
@@ -4002,8 +4003,8 @@ typedef struct
    stbi__uint16 firstcode[16];
    int maxcode[17];
    stbi__uint16 firstsymbol[16];
-   stbi_uc  size[288];
-   stbi__uint16 value[288];
+   stbi_uc  size[STBI__ZNSYMS];
+   stbi__uint16 value[STBI__ZNSYMS];
 } stbi__zhuffman;
 
 stbi_inline static int stbi__bitreverse16(int n)
@@ -4134,7 +4135,7 @@ static int stbi__zhuffman_decode_slowpath(stbi__zbuf *a, stbi__zhuffman *z)
    if (s >= 16) return -1; // invalid code!
    // code size is s, so:
    b = (k >> (16-s)) - z->firstcode[s] + z->firstsymbol[s];
-   if (b >= sizeof (z->size)) return -1; // some data was corrupt somewhere!
+   if (b >= STBI__ZNSYMS) return -1; // some data was corrupt somewhere!
    if (z->size[b] != s) return -1;  // was originally an assert, but report failure instead.
    a->code_buffer >>= s;
    a->num_bits -= s;
@@ -4331,7 +4332,7 @@ static int stbi__parse_zlib_header(stbi__zbuf *a)
    return 1;
 }
 
-static const stbi_uc stbi__zdefault_length[288] =
+static const stbi_uc stbi__zdefault_length[STBI__ZNSYMS] =
 {
    8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8, 8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
    8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8, 8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
@@ -4377,7 +4378,7 @@ static int stbi__parse_zlib(stbi__zbuf *a, int parse_header)
       } else {
          if (type == 1) {
             // use fixed code lengths
-            if (!stbi__zbuild_huffman(&a->z_length  , stbi__zdefault_length  , 288)) return 0;
+            if (!stbi__zbuild_huffman(&a->z_length  , stbi__zdefault_length  , STBI__ZNSYMS)) return 0;
             if (!stbi__zbuild_huffman(&a->z_distance, stbi__zdefault_distance,  32)) return 0;
          } else {
             if (!stbi__compute_huffman_codes(a)) return 0;

stb_image_write.h

@@ -397,7 +397,7 @@ static void stbiw__putc(stbi__write_context *s, unsigned char c)
 
 static void stbiw__write1(stbi__write_context *s, unsigned char a)
 {
-   if (s->buf_used + 1 > sizeof(s->buffer))
+   if ((size_t)s->buf_used + 1 > sizeof(s->buffer))
       stbiw__write_flush(s);
    s->buffer[s->buf_used++] = a;
 }
@@ -405,7 +405,7 @@ static void stbiw__write1(stbi__write_context *s, unsigned char a)
 static void stbiw__write3(stbi__write_context *s, unsigned char a, unsigned char b, unsigned char c)
 {
    int n;
-   if (s->buf_used + 3 > sizeof(s->buffer))
+   if ((size_t)s->buf_used + 3 > sizeof(s->buffer))
       stbiw__write_flush(s);
    n = s->buf_used;
    s->buf_used = n+3;