Diagnostic for #1874, not a fix. The keep-alive widget repaints at 10 Hz
for the whole player lifetime on Linux, which is a periodic compositor
perturbation next to the plane's HDR surface description; whether it
drives the reporter's every-few-seconds colour flip needs an A/B.
Launching with PLEZY_AB_NO_KEEP_ALIVE=1 runs the inert leg of that test
with an otherwise identical build. Revert once the oscillation is
diagnosed.
edde746
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