The lock screen re-rendered and re-committed every output's full
screen-sized cairo surface on every 16ms timer tick regardless of
whether anything had changed, burning 60%+ of a core the entire time
the session was locked.
Track a dirty flag per surface and render only when state actually
changes: a keystroke, a system-status change, the clock minute rolling
over, or an in-flight animation. update() now reports whether it
redrew, so the timer commits only the surfaces that changed.
Also fix the fade-in never formally completing. The eased alpha
approaches 1.0 asymptotically while the 0.001 step throttle suppresses
the final sub-threshold increments, leaving fade_alpha stuck just under
1.0. Since "fade_alpha < 1.0" is the "still animating" signal, that kept
it permanently true and forced a full render every frame. Snap alpha to
exactly 1.0 once the fade duration elapses so the animation completes.
Idle CPU while locked drops from 60%+ to ~2%.
Joseph Dunn
e9550f71f5