This fixes SDL mouse events processing:
- GetRelativeMouseState() always returns the last position, so when the
  polling loop gets several mouse events in one go, we would send
  useless 'no move' events, let's avoid that.
- So as to make sure we don't miss any mouse click / double click, we
  should not use GetRelativeMouseState() to get the button state, but
  rather keep records of the button state ourselves (I've requested SDL
  developers to provide it directly in the event in SDL 1.3).
- bev->state doesn't contain the button state but whether the event is a press
  or a release. Use bev->button instead.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>