Summary:
From Wayland documentation:
"When a seat's focus enters a surface, the pointer image is undefined and
a client should respond to this event by setting an appropriate pointer
image with the set_cursor request."

KWin's interpretation so far for the undefined pointer image was to
remove the pointer image when entering a surface waiting for the client
to set a cursor image. This can result in a short flicker as there might
be a frame without a cursor image.

This patch changes the behavior by keeping the previous image till the
application set a new one. This brings some advantages:
 * if the application is not responding a cursor is still shown
 * if the same cursor is used as in the previous window we don't have a
flicker

CCBUG: 393639

Test Plan: I cannot see the flicker, so only tested with the adjusted tests

Reviewers: #kwin, #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12631