Depending on the platform and device, scroll deltas may either
be represented as pixel deltas specifying the amount
in pixels to scroll or they may be expressed in 'lines' or 'chunks'
for low resolution devices (eg. a traditional mouse wheel).
Pixel deltas are currently available on OS X. X11 currently
supports only integer line deltas, though pixel deltas
are available via XInput2. Windows supports fractional
line deltas.
Add a new api, window.set_cursor, for setting the cursor. The enum MouseCursor lists the possible cursors.
Only X11 is implemented. On OSX, Android, & Win32 the window.set_cursor function
either does nothing or calls the "unimplemented!" macro.
