Add test for mipmap filters

[greeble-dev]

Objective

Add a test for mipmap filters, and as a bonus test GltfLoaderSettings::override_sampler. Neither of these are covered by current tests or examples.

Solution

gltf_override_sampler.mp4

The test supports CI screenshots, although I haven't added it to the CI config.

Problems

It's currently broken.

• The test works by loading the same glTF multiple times, each time changing GltfLoaderSettings::override_sampler.
• That falls afoul of Gltf assets do not reload correctly #18267.
• I've confirmed that the test does work if the assets are duplicated. This can be reproduced in the PR branch by syncing to faefb8dd.

Combining two tests into one is debatable.

• Testing GltfLoaderSettings::override_sampler is convenient and provides a test case for Gltf assets do not reload correctly #18267.
• But makes it harder to isolate bugs.
• And gltf tests should aim to be headless.
• Maybe this is ok for now but gets refactored later.

CI screenshot boilerplate.

• Screenshot supports feels a bit boilerplate-y. It's fairly similar to testbed/helpers.rs.
• My guess is that this whole test will eventually get replaced by a BSN scene.
• Or maybe the CI testing could have a helper that queues up N screenshots and sends an event for each one.

Testing

cargo run --example gltf_override_sampler
cargo run --example test_mipmap_filter --features bevy_ci_testing

Tested on Win10/Nvidia, across Desktop/Vulkan, Chrome/WebGL, Chrome/WebGPU.

[alice-i-cecile]

The test is not currently suitable for screenshot comparison.

We should try and fix this.

Should this test be framed as a glTF test or a renderer test?

Renderer test if we're going to be doing rendering and screenshots. Like you say, gltf loading should be tested purely in headless code.

[greeble-dev]

Test has been redone to focus on rendering - glTF testing is now just a bonus. Also added CI support.

In the longer-term I'm guessing this test should be redone as BSN scenes, and the glTF part would be split off and made headless. But I'm hoping the current choices are acceptable for the short-term.