fix compiler warnings and const error

stb_image.h

@@ -6336,7 +6336,7 @@ static stbi_uc *stbi__process_gif_raster(stbi__context *s, stbi__gif *g)
 
 // this function is designed to support animated gifs, although stb_image doesn't support it
 // two back is the image from two frames ago, used for a very specific disposal format
-static stbi_uc *stbi__gif_load_next(stbi__context *s, stbi__gif *g, int *comp, int req_comp, stbi_uc *two_back)
+static stbi_uc *stbi__gif_load_next(stbi__context *s, stbi__gif *g, int *comp, int, stbi_uc *two_back)
 {
    int dispose; 
    int first_frame; 
@@ -6560,7 +6560,7 @@ static void *stbi__load_gif_main(stbi__context *s, int **delays, int *x, int *y,
    }
 }
 
-static void *stbi__gif_load(stbi__context *s, int *x, int *y, int *comp, int req_comp, stbi__result_info *ri)
+static void *stbi__gif_load(stbi__context *s, int *x, int *y, int *comp, int req_comp, stbi__result_info *)
 {
    stbi_uc *u = 0;
    stbi__gif g;

stb_image_write.h

@@ -985,14 +985,14 @@ static unsigned char stbiw__paeth(int a, int b, int c)
 }
 
 // @OPTIMIZE: provide an option that always forces left-predict or paeth predict
-static void stbiw__encode_png_line(unsigned char *pixels, int stride_bytes, int width, int height, int y, int n, int filter_type, signed char *line_buffer)
+static void stbiw__encode_png_line(const unsigned char *pixels, int stride_bytes, int width, int height, int y, int n, int filter_type, signed char *line_buffer)
 {
    static int mapping[] = { 0,1,2,3,4 };
    static int firstmap[] = { 0,1,0,5,6 };
    int *mymap = (y != 0) ? mapping : firstmap;
    int i;
    int type = mymap[filter_type];
-   unsigned char *z = pixels + stride_bytes * (stbi__flip_vertically_on_write ? height-1-y : y);
+   const unsigned char *z = pixels + stride_bytes * (stbi__flip_vertically_on_write ? height-1-y : y);
    int signed_stride = stbi__flip_vertically_on_write ? -stride_bytes : stride_bytes;
    for (i = 0; i < n; ++i) {
       switch (type) {
@@ -1018,7 +1018,7 @@ static void stbiw__encode_png_line(unsigned char *pixels, int stride_bytes, int
    }
 }
 
-unsigned char *stbi_write_png_to_mem(unsigned char *pixels, int stride_bytes, int x, int y, int n, int *out_len)
+unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int stride_bytes, int x, int y, int n, int *out_len)
 {
    int force_filter = stbi_write_force_png_filter;
    int ctype[5] = { -1, 0, 4, 2, 6 };
@@ -1109,7 +1109,7 @@ STBIWDEF int stbi_write_png(char const *filename, int x, int y, int comp, const
 {
    FILE *f;
    int len;
-   unsigned char *png = stbi_write_png_to_mem((unsigned char *) data, stride_bytes, x, y, comp, &len);
+   unsigned char *png = stbi_write_png_to_mem((const unsigned char *) data, stride_bytes, x, y, comp, &len);
    if (png == NULL) return 0;
 #ifdef STBI_MSC_SECURE_CRT
    if (fopen_s(&f, filename, "wb"))
@@ -1128,7 +1128,7 @@ STBIWDEF int stbi_write_png(char const *filename, int x, int y, int comp, const
 STBIWDEF int stbi_write_png_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const void *data, int stride_bytes)
 {
    int len;
-   unsigned char *png = stbi_write_png_to_mem((unsigned char *) data, stride_bytes, x, y, comp, &len);
+   unsigned char *png = stbi_write_png_to_mem((const unsigned char *) data, stride_bytes, x, y, comp, &len);
    if (png == NULL) return 0;
    func(context, png, len);
    STBIW_FREE(png);