Make sure OpenGL Context is valid before deleting shader

Summary:
Deleting the lanczos filter deletes it's GLShader, this calls
glDeleteProgram

glFooBar always needs to have an openGL context, we don't know we have
this on a screen changed event as it is called from outside the normal
render methods.

BUG: 384884

Test Plan:
Ran on my wayland session. Switched geometry a lot, couldn't reproduce the crash.
Ran on my desktop session, seemed the same as before

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: graesslin, plasma-devel, kwin, KWin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7888