shiny/driver: (X11) handle mouse.ButtonWheel{Left|Right}; send DirStep.

Also handle mouse.ButtonWheel{Up|Down} for the gldriver X11 flavor.

Change-Id: Ib2fd8ae4ae3ea93a37b0ae1ea40594ce1c3c9b46
Reviewed-on: https://go-review.googlesource.com/24635
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed