Getting this crash when using picture-in-picture with WR+SWGL

Reproduces 100% on Linux at least, I haven't tried elsewhere.

Crash report: https://crash-stats.mozilla.org/report/index/01d4caa0-95ea-4ec4-9c80-3746e0201106

MOZ_CRASH Reason: called `Option::unwrap()` on a `None` value

Top 10 frames of crashing thread:

0 libxul.so RustMozCrash mozglue/static/rust/wrappers.cpp:17
1 libxul.so mozglue_static::panic_hook mozglue/static/rust/lib.rs:89
2 libxul.so core::ops::function::Fn::call /builds/worker/fetches/rustc/lib/rustlib/src/rust/library/core/src/ops/function.rs:70
3 libxul.so std::panicking::rust_panic_with_hook library/std/src/panicking.rs:573
4 libxul.so std::panicking::begin_panic_handler::{{closure}} library/std/src/panicking.rs:476
5 libxul.so std::sys_common::backtrace::__rust_end_short_backtrace library/std/src/sys_common/backtrace.rs:153
6 libxul.so rust_begin_unwind library/std/src/panicking.rs:475
7 libxul.so core::panicking::panic_fmt library/core/src/panicking.rs:85
8 libxul.so core::panicking::panic library/core/src/panicking.rs:50
9 libxul.so webrender_bindings::swgl_bindings::SwCompositor::queue_composite gfx/webrender_bindings/src/swgl_bindings.rs:954

https://hg.mozilla.org/mozilla-central/rev/be565446b7d6

https://hg.mozilla.org/mozilla-central/rev/6e726f9e1812