Fix warnings about "conversion to a greater size" that appear in VS2015 in x64 with /W4.

The warning concerns the return value of stbi_err, which is an int, being converted to a pointer. In VS2015 it seems casting directly from a 32-bit int to a 64-bit pointer triggers this warning. Worked around by first converting to a 64-bit int (here size_t) and then to a pointer.

stb_image.h

@@ -902,8 +902,8 @@ static void *stbi__malloc(size_t size)
    #define stbi__err(x,y)  stbi__err(x)
 #endif
 
-#define stbi__errpf(x,y)   ((float *) (stbi__err(x,y)?NULL:NULL))
-#define stbi__errpuc(x,y)  ((unsigned char *) (stbi__err(x,y)?NULL:NULL))
+#define stbi__errpf(x,y)   ((float *)(size_t) (stbi__err(x,y)?NULL:NULL))
+#define stbi__errpuc(x,y)  ((unsigned char *)(size_t) (stbi__err(x,y)?NULL:NULL))
 
 STBIDEF void stbi_image_free(void *retval_from_stbi_load)
 {