simplify rgb2hsv hue calculation

"fmod(x, y)" was replaced by "x - (int)(x / y) * y", and then simplified.

h = fmod((h / 6.0 + 1.0), 1.0);
h = (h / 6.0 + 1.0) - (int)((h / 6.0 + 1.0) / 1.0) * 1.0;
h = (h / 6.0 + 1.0) - (int)(h / 6.0 + 1.0);

src/libImaging/Convert.c

@@ -311,7 +311,8 @@ rgb2bgr24(UINT8 *out, const UINT8 *in, int xsize) {
 
 static void
 rgb2hsv_row(UINT8 *out, const UINT8 *in) {  // following colorsys.py
-    float h, s, rc, gc, bc, cr;
+    double h;
+    float s, rc, gc, bc, cr;
     UINT8 maxc, minc;
     UINT8 r, g, b;
     UINT8 uh, us, uv;
@@ -340,6 +341,8 @@ rgb2hsv_row(UINT8 *out, const UINT8 *in) {  // following colorsys.py
         }
         // incorrect hue happens if h/6 is negative.
         h = fmod((h / 6.0 + 1.0), 1.0);
+        //h = h / 6.0 + 1.0;
+        //h = h - (int)h;
 
         uh = (UINT8)CLIP8((int)(h * 255.0));
         us = (UINT8)CLIP8((int)(s * 255.0));