Make WindowPixmap::isValid virtual and override in concrete implementation

Summary:
If a buffer gets destroyed the texture created from it is still valid.
In such a situation the OpenGLWindowPixmap should return true for isValid
and not false as it did. Similar in QPainter compositor the pixmap is
valid if there is an image copied from the buffer.

This change ensures that for example minimizing an XWayland window
still has a texture during the minimize animation.

BUG: 368440

Test Plan:
Minimize animation plays for X windows and minimized windows
are shown in present windows.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2724