STB: Fixed warning.

3rdparty/stb/stb_image.c

@@ -3106,7 +3106,7 @@ static void stbi__YCbCr_to_RGB_simd(stbi_uc *out, stbi_uc const *y, stbi_uc cons
    // so just accelerate step == 4 case.
    if (step == 4) {
       // this is a fairly straightforward implementation and not super-optimized.
-      __m128i signflip  = _mm_set1_epi8(-0x80);
+      __m128i signflip  = _mm_set1_epi8((char)0x80);
       __m128i cr_const0 = _mm_set1_epi16(   (short) ( 1.40200f*4096.0f+0.5f));
       __m128i cr_const1 = _mm_set1_epi16( - (short) ( 0.71414f*4096.0f+0.5f));
       __m128i cb_const0 = _mm_set1_epi16( - (short) ( 0.34414f*4096.0f+0.5f));