Make GL Context current when deleting lanczos textures in timer handler
A timer could have fired at any time. We process mulitple QtQuickViews on timers which change the GL context.
Deleting a kwin GLTexture calls glDeleteTextures/glDeleteFramebuffers which need an active context.
Surprisingly I haven't directly seen a crash report from this, but it doesn't seem right.
Edited by David Edmundson