stb_image_write: STBI_WINDOWS_UTF8 -> STBIW_WINDOWS_UTF8

Fixes issue #925.

stb_image_write.h

@@ -178,7 +178,7 @@ STBIWDEF int stbi_write_tga(char const *filename, int w, int h, int comp, const
 STBIWDEF int stbi_write_hdr(char const *filename, int w, int h, int comp, const float *data);
 STBIWDEF int stbi_write_jpg(char const *filename, int x, int y, int comp, const void  *data, int quality);
 
-#ifdef STBI_WINDOWS_UTF8
+#ifdef STBIW_WINDOWS_UTF8
 STBIWDEF int stbiw_convert_wchar_to_utf8(char *buffer, size_t bufferlen, const wchar_t* input);
 #endif
 #endif
@@ -285,7 +285,7 @@ static void stbi__stdio_write(void *context, void *data, int size)
    fwrite(data,1,size,(FILE*) context);
 }
 
-#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBIW_WINDOWS_UTF8)
 #ifdef __cplusplus
 #define STBIW_EXTERN extern "C"
 #else
@@ -303,7 +303,7 @@ STBIWDEF int stbiw_convert_wchar_to_utf8(char *buffer, size_t bufferlen, const w
 static FILE *stbiw__fopen(char const *filename, char const *mode)
 {
    FILE *f;
-#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBIW_WINDOWS_UTF8)
    wchar_t wMode[64];
    wchar_t wFilename[1024];
    if (0 == MultiByteToWideChar(65001 /* UTF8 */, 0, filename, -1, wFilename, sizeof(wFilename)/sizeof(*wFilename)))