eframe: Improve `glow` context switching #12128

Job	Run time
android	23s
cargo-deny aarch64-apple-darwin	39s
Check wasm32 + wasm-bindgen	5m 37s
cargo-deny aarch64-linux-android	38s
Format + check + test	5m 29s
Check Windows	1m 56s
cargo-deny i686-pc-windows-gnu	39s
cargo-deny i686-pc-windows-msvc	43s
cargo-deny i686-unknown-linux-gnu	38s
cargo-deny wasm32-unknown-unknown	37s
cargo-deny x86_64-apple-darwin	41s
cargo-deny x86_64-pc-windows-gnu	42s
cargo-deny x86_64-pc-windows-msvc	37s
cargo-deny x86_64-unknown-linux-gnu	43s
cargo-deny x86_64-unknown-linux-musl	41s
cargo-deny x86_64-unknown-redox	38s
	21m 21s

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