mouse synthesis: don't say the left button is pressed during a release

Follows the same reasoning as Iefe63cd753f9f8bb04278fd04a4d728e3deda25e
in qtbase: buttons should have only the buttons which are still pressed.

Change-Id: I894df43254eb1b1ce173fd7acc7190e9765beb48
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>