Improve Button performance

[ericoporto]

ImGi_Res ImGi_Context::Button(String label, ImGi_Icon icon, ImGi_Opt opt)
{
  ImGi_Res res = eImGi_Res_None;
  ImGi_ID id;
  if(String.IsNullOrEmpty(label)) {
    id = this.GetID(String.Format("%d", icon));
  }
  else
  {
    id = this.GetID(label);
  }
  Rect* r = this.layout_next();
  this.update_control(id, r, opt);
  /* handle click */
  if(this.mouse_pressed == eImGi_Mouse_Left && this.focus == id)
  {
    res |= eImGi_Res_Submit;
  }
  /* draw */
  this.draw_control_frame(id, r, eImGi_Col_Button, opt);
  if(!String.IsNullOrEmpty(label)) this.draw_control_text(label, r, eImGi_Col_Text, opt);
  if(icon) this.draw_icon(icon, r, this.style.colors[eImGi_Col_Text]);
  return res;
}

The button source code is not slow, but could be faster, we want to be able to handle hundreds of buttons. For now around 40 may already slow down things.

[ericoporto]

Apparently the most expensive part is the draw_control_frame which spawns 5 rectangles :/

[ericoporto]

If the size of the button is small enough - say, width smaller than 50px, it may be best to draw two rectangles, one for the border+inner with border color, and then one for the inner part on top, instead of drawing 1 for each line and then the insides.