gdk: update csw event mask filter to handle smooth scroll

gdk: update csw event mask filter to handle smooth scroll

Events of type GDK_SCROLL will be received if the client side window
event mask has either GDK_SCROLL_MASK or GDK_SMOOTH_SCROLL_MASK.

GDK_BUTTON_PRESS_MASK has been removed from type_masks[GDK_SCROLL]
as that bit is often set for other-than-scrolling purposes, and
yet have the window receive scroll events. In GTK+, this forces
non-smooth events bubbling, even if the widgets above want smooth
events, and legitimately set GDK_[SMOOTH_]SCROLL_MASK.