fix compiler warnings

stb_image.h

@@ -1750,7 +1750,8 @@ typedef struct
 
 static int stbi__build_huffman(stbi__huffman *h, int *count)
 {
-   int i,j,k=0,code;
+   int i,j,k=0;
+   unsigned int code;
    // build size list for each symbol (from JPEG spec)
    for (i=0; i < 16; ++i)
       for (j=0; j < count[i]; ++j)
@@ -2157,7 +2158,7 @@ stbi_inline static stbi_uc stbi__clamp(int x)
 }
 
 #define stbi__f2f(x)  ((int) (((x) * 4096 + 0.5)))
-#define stbi__fsh(x)  ((x) << 12)
+#define stbi__fsh(x)  ((x) * 4096)
 
 // derived from jidctint -- DCT_ISLOW
 #define STBI__IDCT_1D(s0,s1,s2,s3,s4,s5,s6,s7) \
@@ -4720,7 +4721,7 @@ static void stbi__de_iphone(stbi__png *z)
    }
 }
 
-#define STBI__PNG_TYPE(a,b,c,d)  (((a) << 24) + ((b) << 16) + ((c) << 8) + (d))
+#define STBI__PNG_TYPE(a,b,c,d)  (((unsigned) (a) << 24) + ((unsigned) (b) << 16) + ((unsigned) (c) << 8) + (unsigned) (d))
 
 static int stbi__parse_png_file(stbi__png *z, int scan, int req_comp)
 {