Trim cosmic-text's shape run cache

[UkoeHB]

Objective

• Fixes Use cosmic-text shaping buffer #14991. The cosmic-text shape run cache requires manual cleanup for old text that no longer needs to be cached.

Solution

• Add a system to trim the cache.
• Add an average fps indicator to the text_debug example.

Testing

Tested with cargo run --example text_debug.

• No shape run cache: 82fps with ~1fps variance.
• Shape run cache no trim: 90-100fps with ~2-4fps variance
• Shape run cache trim age = 1: 90-100fps with ~2-8fps variance
• Shape run cache trim age = 2: 90-100fps with ~2-4fps variance
• Shape run cache trim age = 2000: 80-120fps with ~2-6fps variance

The shape run cache seems to increase average FPS but also increases frame time variance (when there is dynamic text).

[alice-i-cecile]

A comment here on why the trim value was chosen would be really helpful.

[alice-i-cecile]

Good investigation. Based on those numbers, I think "shape run cache no trim" is my preferred setting here, but I'm open to being convinced otherwise.

[UkoeHB]

Based on those numbers, I think "shape run cache no trim" is my preferred setting here, but I'm open to being convinced otherwise.

With no trimming whatsoever the cache has unbounded growth.