gpui: Add support for slash pattern fills (///)

[iamnbutler]

TODO:

• Add BackgroundTag::PatternSlash
• Support metal slash pattern fills
• Support blade slash pattern fills

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Adds support for a new background type in gpui, pattern_slash.

Usage:

div().size(px(56.0)).bg(pattern_slash(gpui::red()))

This will create a 56px square with a red slash pattern fill.

You can run the pattern example with cargo run -p gpui --example pattern:

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After talking with @as-cii at length about how we want to support patterns in gpui, we decided for now we'll simply add a new BackgroundTag specific to this pattern.

It isn't the best long term plan however – we'll likely want to introduce the concept of a Fill at some point so we can have Fill::Solid, Fill::Gradient(LinearGradient), etc in the future.

The pattern is designed to seamlessly tile vertically for elements of the same height. For example, for use in editor line backgrounds:

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Release Notes:

(do we do gpui release notes?)

• Adds support for slash pattern fills in gpui.

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[iamnbutler]

@jansol Mind taking a peek at this one? Any thoughts?

[iamnbutler]

Took a stab at the blade version. While I was doing that I noticed at larger scales there is a bit of mismatch at larger scales:

[jansol]

Looks like the base pattern size does not take display scaling into account? Other than that nothing immediately caught my eye on a quick peek. This was a problem for squiggly underlines too and I can't remember rn if those were ever fixed?

Also it would be neat if it was possible to specify the on and off size of the pattern from app code in em or pt units.

[iamnbutler]

@jansol Thanks for taking a look! Hmm... let me take a look at the display scaling thing.

Agreed. I want to expand patterns quite a bit more than this – at the very least I'd like to use them for dashes. I'll see what I can do then about adding some more control!