Switch GlyphString to logical glyph order

[afishhh]

Story time

So once upon a time a library called "HarfBuzz" was created. This library handled text shaping very well and many serious GUI programs started using it. Said library's main entrypoint is hb_shape which takes a buffer of codepoints along with an array of font features and outputs shaped glyphs in visual order. Here developers of users of HarfBuzz, like subrandr or Chromium, say "okay well I'll keep them in visual order". Unbeknownst to them, they have just fallen prey to impeccable bait laid out by HarfBuzz. In fact, a higher-level layout engine does not benefit from storing glyphs in visual order. The opposite happens: your code is now littered with isLtr() or !self.direction.is_reverse().

Line breaking: happens on the text in logical order.
Hanging white space: the white space at the logical end of a line is hanged.
Splitting the glyph string across spans: if done on logical order glyphs, it will be naturally mirrored on reverse strings.

So yeah just pay the cost of reversing the glyphs and avoid the cost of juggling iteration order later. Having things be clean and logical also ends up simplifying some cases.

Note

I am not the first to realize this, the first thing Firefox does is call hb_buffer_reverse if the direction was right to left.

In other news

Switched GlyphString to use a Vec since fighting with LinkedList without cursors is too much of a pain here, it's probably faster anyway. Also simplified shaping parameter flow and splitting of text across spans.

I'm going to go back to improving white space handling on top of this PR now (which will hopefully make it much simpler). Will finish this once I'm confident this is the right approach. Alternatively I might realize I'm wrong and somehow visual order is better but I doubt that.

[afishhh]

I'm going to go back to improving white space handling on top of this PR now (which will hopefully make it much simpler).

I have a proof-of-concept for white space hanging specifically. It's terrible and bad and horrible and bad but honestly the spec is bad and horrible so I'm not sure it can be made not-horrible. Either way I think storing things in logical order is worth it.

[afishhh]

Maybe I should just merge this instead of being eternally uncertain. The main thing I dislike is that the shaping is slightly uglier but I'm probably overthinking it since this is an overall simplification.

[afishhh]

These shaping changes are bad and broken and bad, found some bugs. Need to find a clean way to do that before this can be merged.

[afishhh]

These shaping changes are bad and broken and bad, found some bugs. Need to find a clean way to do that before this can be merged.

It's now less bad and not obviously broken, maybe not "clean" but at least clean-er. Will merge after more testing.