Since GdkTimeCoord stores only axis values, prior to this change,
if a device didn't report GDK_AXIS_X or GDK_AXIS_Y, the history
attached to merged motion events wouldn't contain any positional
information.

Commit 60122760 already addressed
this issue for devices without tools by storing the event position
in GdkTimeCoord using GDK_AXIS_X and GDK_AXIS_Y and augmenting the
GdkTimeCoord's axis bitmask accordingly.

This change generalizes that workaround to all devices. Note that
if a device DOES report values for GDK_AXIS_X and GDK_AXIS_Y, those
values won't be overwritten.

Closes #4809