Eliminate `vello_common` dependency from `glifo`

Nobody has claimed this yet.

Parley Draw currently depends on `vello_common` for the `Pixmap` type. It uses this as a representation for bitmap data (including in the public API), and also for reading PNG files.

This is not ideal as it couples `glifo` version to specific vello versions. Which will mean publishing a new version of `glifo` every time we publish vello (sparse strips), and also make it awkward to use `glifo` with main-branch versions of vello.

I can think of a few ways to eliminate this dependency:

- Duplicate (copy-paste) `Pixmap` into `glifo`
- Include a simplified version of `Pixmap` in `glifo`
- Move `Pixmap` into a shared crate (peniko? a new crate?)
- Use simple types (e.g. `Vec`) to represent bitmap data in `glifo`

What do other people think the best way to deal with this is? (CC: @grebmeg @valadaptive @taj-p @LaurenzV)

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Note: Parley Draw also pulls in `vello_common` as part of `vello_cpu` when that feature is enabled. But that dependency is optional and doesn't impact anyone who doesn't have the `vello_cpu` feature enabled (and therefore not problematic in my opinion).

Start by reviewing glifo's public Pixmap API and its PNG-reading code, then compare the dependency-removal options described in the issue. Done means glifo no longer depends on vello_common while its bitmap representation and PNG support remain usable.

Written by the indexing model from the issue text.