vello_common: Replace external guillotiere with built-in no_std-compatible port

[grebmeg]

The glyph rendering path in parley_draw I've been working on depends on multi_atlas and image_cache for atlas allocation. Since guillotiere requires std (via euclid/num-traits), these modules were gated behind cfg(feature = "std"), which would force the entire glyph rendering pipeline to be std-gated as well. Porting the allocator to no_std removes this constraint and keeps the glyph rendering path available in no_std builds.

This PR imports and modifies guillotiere's tree-based guillotine algorithm directly into allocator.rs, replacing euclid types with local Size/Point/Rect equivalents and swapping std imports for core/alloc.

We discussed in one of the recent office hours that it would be fine to add the simpler shelf allocator under vello_common/. However, since my glyph caching is already based on the ability to deallocate, I thought there was no harm in continuing to use the current algorithm. I have not added this to upstream because I am not aware of the plan there or the current review capacity, but I am happy to do so if we decide that is the right approach.

[grebmeg]

Hey @LaurenzV, I think I’d prefer to merge this for now, and then once servo/euclid#543 is merged and guillotiere is updated, we can switch the current usage to the official repo.

[LaurenzV]

LGTM, double-checked by copy-pasting the code snippets from guillotiere and wasn't able to find any relevant discrepancies.