shiny/driver/internal/win32: use relative coordinates for mouse wheel events

Windows mouse wheel events report the cursor position in
absolute screen coordinates, while other mouse event's positions
are relative to the window.
This adds ScreenToClient to syscall_windows.go and converts the
event's position for mouse wheel events.

Fixes golang/go#18695

Change-Id: Ic086033e435cf4699c7ff0aad90e1a1ce233bb02
Reviewed-by: Alex Brainman <>
3 files changed