From: Max Horn
Subject: Modifier key fix

The internal modifier state can get out of sync with reality. To
trigger this, do for example this:
  1) Launch an SDL app
  2) Alt-click on the desktop (this will hide the SDL app).
  3) Bring the SDL app back to the front
  4) SDL will still think alt is pressed (and as such will treat left
clicks like middle clicks). If you press and release alt, it'll be fine
again.

The attached patch cures this by rechecking the modifier state whenever
we process an event.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%40817