Set delta_x or delta_y for GdkScrollEvent.
HIWORD (wParam) in WM_MOUSE(H)WHEEL is the scroll delta.
A delta value of WHEEL_DELTA (which is 120) means scrolling
one full unit of something (for example, a line).
The delta should also be multiplied by the value that the
SystemParametersInfo (SPI_GETWHEELSCROLL(LINES|CHARS), 0, &value, 0)
call gives back, unless it gives back 0xffffffff, in which case
it indicates that scrolling is page- or screen-based, not line-based
(GDK doesn't support that at the moment).
Also, all deltas should be inverted, since MS sends negative deltas
when scrolling down (rotating the wheel back, in the direction of
the user).
With deltas set the mode should be set to GDK_SCROLL_SMOOTH.
Fixes issue 1263.
359df028be