gpui_perf: measure what window composition costs

[sunli829]

Adds --composition to the showcase, which composes its window around an empty native surface as one embedding a webview does, and reports the time spent presenting each frame (LayoutStats::present_time, test-support only). With composition, presenting includes splitting the scene per surface.

This answers whether enabling window composition (as Allsum does for its webview) makes drawing slower: run the showcase with and without --composition and compare the present column.

• fast/composition.rs times present_scene(); fast/stats.rs adds present_time and FramePhaseTimes::add_present.
• gpui_perf showcase: --composition flag, present column in the report.
• CONTRIBUTING.md: a "Window composition" section on how to measure it.

script/check-upstream: 0 violations. cargo test -p gpui --features test-support --lib fast:: and cargo test -p gpui_perf pass.

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