Cleanup some bevy_text pipeline.rs

[nicopap]

Objective

• bevy_text/src/pipeline.rs had some crufty code.

Solution

Remove the cruft.

• &mut self argument was unused by TextPipeline::create_text_measure, so we replace it with a constructor TextMeasureInfo::from_text.
• We also pass a &Text to from_text since there is no reason to split the struct before passing it as argument.
• from_text also checks beforehand that every Font exist in the Assets. This allows rust to skip the drop code on the Vecs we create in the method, since there is no early exit.
• We also remove the scaled_fonts field on TextMeasureInfo. This avoids an additional allocation. We can re-use the font on fonts instead in compute_size. Building a ScaledFont seems fairly cheap, when looking at the ab_glyph internals.
• We also implement ToSectionText on TextMeasureSection, this let us skip creating a whole new Vec each time we call compute_size.
• This let us remove compute_size_from_section_text, since its only purpose was to not have to allocate the Vec we just made redundant.
• Make some immutabe Vec<T> into Box<[T]> and String into Box<str>
• {min,max}_width_content_size fields of TextMeasureInfo have name width in them, yet the contain information on both width and height.
• TextMeasureInfo::linebreak_behaviour -> linebreak_behavior

Migration Guide

• The ResMut<TextPipeline> argument to measure_text_system doesn't exist anymore. If you were calling this system manually, you should remove the argument.
• The {min,max}_width_content_size fields of TextMeasureInfo are renamed to min and max respectively
• Other changes to TextMeasureInfo may also break your code if you were manually building it. Please consider using the new TextMeasureInfo::from_text to build one instead.
• TextPipeline::create_text_measure has been removed in favor of TextMeasureInfo::from_text

[ickshonpe]

Seem like good changes. Just getting #7779 working and fixing all the bugs took so much effort, I didn't want to work on it any more and some of the code I left was really ugly and embarrassing, and I'm relieved it's getting fixed now.

I've been doing some work on TextPipeline too, fixing some bugs, but there shouldn't be much conflict.
I'll do a proper review after I've finished my PRs.

[ickshonpe]

Ah just noticed this from the changelog, it's not a mistake that these were called min_content_width and max_content_width. The word width here isn't referring to the result but the constraint the size value is based on. min_content_width is the space (horizontal and vertical) that the text will occupy given a zero width constraint (in which case the text wraps at every breakpoint) and max_content_width is the space the text will occupy given an infinite width constraint (in which case the text next never wraps).

Edit: didn't mean to start a review just to leave a comment 😓