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/dynapi/SDL_dynapi.sym

@@ -859,6 +859,8 @@ SDL3_0.0.0 {
     SDL_wcsncasecmp;
     SDL_wcsncmp;
     SDL_wcsstr;
+    SDL_modf;
+    SDL_modff;
     # extra symbols go here (don't modify this line)
   local: *;
 };