fix draw text with alpha

[var4yn]

My pull request about problem.

Added the HAS_ALPHA: bool associated constant to the Pixel trait.
I want to update the current has_alpha check in the draw_text_mut() func to use Pixel::HAS_ALPHA once it becomes available.

This requires updating the minimum Rust version to 1.81.0.
and the rayon crate version to 1.10.0.

[var4yn]

I don't know how to fix the safety/miri errors.
The problem is caused by inexact calculations.

[cospectrum]

I don't know how to fix the safety/miri errors. The problem is caused by inexact calculations.

Yeah, it's not your fault. Probably related to the recent nightly update.