gtk: cap FMDSP redraw at 30fps and re-enable client vsyncHEADmastermain

Unthrottled async presenting (previous commit) lets the app's frame
writes race picom's composite read: users see a vertical tear inside
the FMDSP area about once every few seconds. Re-enabling client vsync
alone brings back the 47fps missed-vblank judder (33% of frame-clock
ticks at 16.7-33ms). Both fail together.

Fix: keep client vsync ON and throttle queue_draw to 30fps in the
frame-clock tick, so every presented frame lands uniformly at ~33.3ms:
measured 29.6 visible fps, intervals avg 33.8ms, CPU down (app 11%,
Xorg 11%, picom 3.5%), no stalls. FM_FPS=60 restores full-rate drawing,
FM_NO_VSYNC=1 restores the async experiment.

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