…ling and log gpu errors. (flutter#169378)

Attempted fix for flutter#166668 .
Plan is to ask for testing once rolled into g3 since we weren't given a
repro.

Explaination: we can only submit command buffers to the metal command
queue when the device is in the foreground. push notifications
temporarily wake up the app and can trigger images to begin decoding.
This decoding can fail, possibly due to the app re-backgrounding. In
theory, the sync switch should prevent this from happening - but because
we don't actually block on the scheduling of the command buffer there is
technically a race that can happen.

Add the ability to block submission on scheduling for metal. note that
other graphics APIs do not have a distinction for commited vs scheduled
and so it no-ops for them.


See
https://developer.apple.com/documentation/metal/preparing-your-metal-app-to-run-in-the-background

> When UIKit calls your app delegate’s
[applicationDidEnterBackground(_:)](https://developer.apple.com/documentation/UIKit/UIApplicationDelegate/applicationDidEnterBackground(_:))
method, make sure Metal has scheduled all command buffers you’ve already
committed before your app returns control to the system. ... Finish
encoding commands to render the frame and commit the command buffer,
then call
[waitUntilScheduled()](https://developer.apple.com/documentation/metal/mtlcommandbuffer/waituntilscheduled()).