Ell authored May 04, 2018 and Michael Natterer committed May 20, 2018

In gimp_display_shell_canvas_tool_events(), use
gdk_window_set_event_compression(), instead of implementing our own
motion compression, which used to introduce all sorts of weird
effects when combined with extended input devices, that we had to
hackishly work around.

For tools that use GIMP_MOTION_MODE_EXACT, we call
gdk_window_set_event_compression() to disable motion compression
for the canvas window upon initializing the tool in response to a
GDK_BUTTON_PRESS event, and again to re-eanble compression upon the
corresponding GDK_BUTTON_RELEASE event.

This commit also merges
gimp_display_shell_canvas_tool_events_internal() back into
gimp_display_shell_canvas_tool_events().  The split was a detail
of our custom motion compression implementation.