Fetch current_epoch via render_api

[shubhamg13]

This PR targets to access the Scene's PipelineInfo using render_api.

Aiming render_api as only interface used to communicate with WebRender.
As of now render_api is already being used for most of communication except this one (and update ).

In this PR

1. pipeline_info is moved to SceneBuilderThread along with removed_pipelines.
2. Connecting PublishPipelineInfo to SceneBuilerThread.
3. Along with making this pipeline_info synchronous as earlier we need to rely on calling of renderer.update() to PublishPipelineInfo. Now using event loop of SceneBuilerThread it is synchronous.

Add-ons
4. FlushPipelineInfo is also connected to SceneBuilerThread as required.

[shubhamg13]

@mrobinson @xiaochengh Please take a look.

[yezhizhen]

Let's fix the CI first. Can you explain the motivation of "synchronous", and how is it achieved?

[shubhamg13]

Let's fix the CI first. Can you explain the motivation of "synchronous", and how is it achieved?

Please run the workflow again. I updated the PR description. PTAL.

[xiaochengh]

Could you elaborate on the motivation of this PR?

Why do we prefer a sync api over an async one? Is there any issue with the current async API? Or do you just want to make it easier to call (which on its own might not justify the change though...)?

[shubhamg13]

Could you elaborate on the motivation of this PR?

Why do we prefer a sync api over an async one? Is there any issue with the current async API? Or do you just want to make it easier to call (which on its own might not justify the change though...)?

I updated the PR description. Please take a look.

[mrobinson]

I'm also curious about the motivations here. I don't really great about making large architectural changes to WebRender. It's better to have fewer changes on top of the upstream version. The exception is for solving a problem that we are encountering. In that case, we should discuss with the folks at Mozilla about making our changes upstream or look at reworking the code in Servo.

[mrobinson]

Regarding this change: we try to keep our modifications to WebRender as minimal as possible and limited just to things that Servo needs. If you feel like this could be generally useful, a good step might be to submit it upstream to the Gecko repository and see if it's something that also makes sense for Firefox as well. If it lands upstream, it will also be available the next time we re-sync WebRender with upstream.