shiny/driver/internal/win32: actually send mouse events

At this moment mouse events are only sent when any mouse
button is down. Change this to always send mouse event.

Suggested by Nigel.

Change-Id: I42fe1cb3e7eeecc7eec96bc9356711167e1f5658
Reviewed-on: https://go-review.googlesource.com/23120
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed