Improved text batching (#14848)

# Objective

The UI text rendering is really slow because it extracts each glyph as a
separate ui node even though all the glyphs in a text section have the
same texture, color and clipping rects.

## Solution

Store the glyphs in a seperate contiguous array, queue one transparent
ui item per text section which has indices into the glyph array.

## Testing

```cargo run --example many_glyphs --release```

Runs at about 22fps on main and 95fps with this PR on my computer.

I'll do some proper comparisons once I work out why tracy 11 is refusing to run.

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Co-authored-by: Kristoffer Søholm <k.soeholm@gmail.com>