shiny/driver/gldriver: (cocoa) don't set a mouse button for drags.
shiny/driver/gldriver: (cocoa) don't set a mouse button for drags.

This fixes the example/fluid program.

Change-Id: Ia97bcb3db8f856f31ba39a2ce9649f6dabb27166
Reviewed-on: https://go-review.googlesource.com/23113
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/exp,golang/exp,golang/exp