We had a ton of issues around `TexturesDelta` that weren't properly
applied because we early-out of some function:
* https://github.com/emilk/egui/pull/8313
* https://github.com/emilk/egui/pull/8250
* https://github.com/emilk/egui/pull/8279
This PR changes texture updates, so that we always store them after
taking them out of `FullOutput` and keep the delta around until it's
actually applied (by passing &mut refs and draining instead of
iterating). So even if we add a new early return somewhere, that can't
break texture updates.
It also optimizes `TexturesDelta::append` by dropping any previous
deltas if there's a new `whole` delta or a `free`.
It also adds a debug assert that any `TexturesDelta` is empty when
dropped, as an additional safeguard in case the bug sneaks back in.
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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
