fix type typo

bmp.c

@@ -113,7 +113,7 @@ void bmp_write(image_t *img, dimensions_t size, char *filename) {
         .color_space_type={"Win "}
     };
 
-    for (ssize_t i = 0; i < size.height; i++) {
+    for (size_t i = 0; i < size.height; i++) {
         for (size_t j = 0; j < size.width; j++) {
             size_t idx = data_size - NUM_COLOR_COMPONENTS * ((i + 1) * size.width - j);
             rgba_color_t color = image_get_pixel(img, i, j);