Don't commit the wl_surface from outside Qt code

Plasma code calling wl_surface::commit outside of Qt's knowledge is
bound to lead to errors. We're going to cause kwin to configure a
surface before Qt has sent initial properties. Potentially causing
protocol violations.

This has the added benefit of blur and sliding being synced to the frame
correctly.

It does mean a user calling enableBlur/slide/backgroundContrast should
trigger a repaint in their code, but in all instances this happens
anyway.