effects: Make OpenGL context current before deleting framebuffer

When we delete a window we unredirect, when we unredirect, we delete the
framebuffer which requires an openGL context.

handleWindowDeleted is the entry point from workspace events to effects
code, so it's made current here.

BUG: 444665
BUG: 471139

src/libkwineffects/kwinoffscreeneffect.cpp

@@ -250,6 +250,7 @@ void OffscreenEffect::handleWindowDamaged(EffectWindow *window)
 
 void OffscreenEffect::handleWindowDeleted(EffectWindow *window)
 {
+    effects->makeOpenGLContextCurrent();
     unredirect(window);
 }