Webrender panic in /css/css-paint-api/background-image-tiled.https.html

[jdm]

 ▶ CRASH [expected FAIL] /css/css-paint-api/background-image-tiled.https.html
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  │ Non-tiled image with no visible images detected! Properties None (thread WRRenderBackend#0, at /root/.cargo/git/checkouts/webrender-c3596abe1cf4f320/ed6ed92/webrender/src/batch.rs:2301)
  │    0: servo::backtrace::print
  │    1: servo::main::{{closure}}
  │    2: std::panicking::rust_panic_with_hook
  │              at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/panicking.rs:530
  │    3: rust_begin_unwind
  │              at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/panicking.rs:437
  │    4: std::panicking::begin_panic_fmt
  │              at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/panicking.rs:391
  │    5: webrender::batch::BatchBuilder::add_prim_to_batch
  │    6: webrender::batch::BatchBuilder::add_pic_to_batch
  │    7: webrender::batch::BatchBuilder::add_prim_to_batch
  │    8: <webrender::render_target::ColorRenderTarget as webrender::render_target::RenderTarget>::build
  │    9: webrender::frame_builder::FrameBuilder::build
  │   10: webrender::render_backend::Document::build_frame
  │   11: webrender::render_backend::RenderBackend::update_document
  │   12: webrender::render_backend::RenderBackend::process_transaction
  │   13: webrender::render_backend::RenderBackend::run
  │   14: std::sys_common::backtrace::__rust_begin_short_backtrace
  │   15: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   16: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │              at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/liballoc/boxed.rs:1074
  │       <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │              at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/liballoc/boxed.rs:1074
  │       std::sys::unix::thread::Thread::new::thread_start
  │              at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/sys/unix/thread.rs:87
  │   17: start_thread
  │   18: __clone
  │ [2020-07-29T16:42:20Z ERROR servo] Non-tiled image with no visible images detected! Properties None
  └ Stack trace for thread "WRRenderBackend#0"

This was introduced when updating webrender in #27207.

[jdm]

This can show up as a TIMEOUT on macOS.