[Impeller] Add a new API 'Context::WaitUntilCommandsCompleted' to ensure that commands are completed.

[ColdPaleLight]

I'm working on flutter/flutter#110829, and I found that Impeller doesn't have an API to ensure that all commands are completed.

Let's take the following code as an example. In the following piece of code, we will draw the DisplayList into a Texture in the Raster thread, and then blit the Texture into the CPU Buffer on the IO thread. So we need to ensure that when we do the blit operation on the IO thread, the commands about the DisplayList drawn to the Texture have been completed in the GPU.

final ui.PictureRecorder recorder = ui.PictureRecorder();
final ui.Canvas canvas = ui.Canvas(recorder);
canvas.drawRect(const Rect.fromLTWH(0.0, 0.0, 100, 100), Paint()..color = Colors.red);
final ui.Picture picture = recorder.endRecording();
ui.Image image = await picture.toImage(100, 100);
ByteData? byteData = await image.toByteData();

Another place where we need to use this new API is "blit texture to buffer". We need to ensure that the blit operation has been executed on the gpu before using the cpu buffer.

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[dnfield]

I don't think this belongs on Context - you want to know when the commands from a particular buffer are completed right?

For example, should this wait for all commands from compute to finish as well?

[ColdPaleLight]

I tried adding a parameter to the SubmitCommands method of CommandBuffer to solve the problem. But I found that in some scenarios, developers can't call SubmitCommands themselves, or even they can't find the Command they want to wait for.

for example

engine/shell/common/snapshot_controller_impeller.cc

Lines 64 to 66 in 4464920

	std::shared_ptr<impeller::Image> image =
	picture.ToImage(*context, render_target_size);
	context->GetContext()->WaitUntilCommandsCompleted();

[ColdPaleLight]

@dnfield Do you have any thoughts on this example of not being able to access the specific CommandsBuffer? Thanks!
Also cc @chinmaygarde

[dnfield]

When you call EntityPass::Render from AiksContext::Render maybe there could be some parameter like a callback for when the texture is done?

[dnfield]

EntityPass::Render knows the command buffer, and the command buffer has SubmitCommands(const CompletionCallback& callback).

[ColdPaleLight]

@dnfield Thank you very much! I file a new PR #37772, Any thoughts?

[ColdPaleLight]

Closing it since there is a better solution #37772