Added SDL_modf() and SDL_modff()

This function is useful for accumulating relative mouse motion if you want to only handle whole pixel movement.
e.g.
static float dx_frac, dy_frac;
float dx, dy;

/* Accumulate new motion with previous sub-pixel motion */
dx = event.motion.xrel + dx_frac;
dy = event.motion.yrel + dy_frac;

/* Split the integral and fractional motion, dx and dy will contain whole pixel deltas */
dx_frac = SDL_modff(dx, &dx);
dy_frac = SDL_modff(dy, &dy);
if (dx != 0.0f || dy != 0.0f) {
    ...
}

src/stdlib/SDL_stdlib.c

@@ -322,6 +322,28 @@ float SDL_log10f(float x)
 #endif
 }
 
+double
+SDL_modf(double x, double *y)
+{
+#if defined(HAVE_MODF)
+    return modf(x, y);
+#else
+    return SDL_uclibc_modf(x, y);
+#endif
+}
+
+float SDL_modff(float x, float *y)
+{
+#if defined(HAVE_MODFF)
+    return modff(x, y);
+#else
+    double double_result, double_y;
+    double_result = SDL_modf((double)x, &double_y);
+    *y = (float)double_y;
+    return (float)double_result;
+#endif
+}
+
 double
 SDL_pow(double x, double y)
 {