XI2.h: remove XI2Mask, add XISetMask and friends.

XI2.h: remove XI2Mask, add XISetMask and friends.

XISetMask, XIClearMask, XIMaskIsSet serve to set, clear or check a bit in
the provided array.
XIMaskLen is a macro to get the minimum length of a mask for a given event
type.

They are expected to be common ways to deal with event masks, i.e. clients
will do:

unsigned char mask[XIMaskLen(XI_ButtonRelease)] = {0};
XISetMask(mask, XI_ButtonPress)
XISetMask(mask, XI_ButtonRelease)

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>