experiment: add pre-redisplay-functions as alternative refresh triggerexperiment/pre-redisplay-functions

Add feature flag (kitty-gfx-use-pre-redisplay) to compare
post-command-hook vs pre-redisplay-functions as the scheduling
trigger for image refresh. Includes instrumentation counters
(trigger count, skipped count) logged per refresh cycle for
quantitative comparison.

Results: pre-redisplay-functions fires ~36x more often than
post-command-hook for identical user activity (3176 vs 89 refresh
cycles in comparable sessions). Emacs redraws far more often than
users run commands (cursor blink, timers, internal redraws), and
pre-redisplay-functions fires per-window on every redisplay.
The current post-command-hook approach remains superior.

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