wgpu should let app wait until SurfaceTexture presentation is complete

[jimblandy]

Is your feature request related to a problem? Please describe.
There's no way to tell when a particular SurfaceTexture is done being presented. This makes it hard for programs to match the frame rate of the swapchain.

Describe the solution you'd like
wgpu should offer a blocking function or future so that the user can wait until it's worth rendering another frame.

Describe alternatives you've considered
The examples hard-code 60Hz, which is unsatisfying. You could present as fast as possible, but in Mailbox mode this will use the full CPU.

Additional context
Possibly relevant:

• Non-blocking API #1451
• Presenting blocks other operations with VSync on #421
• Allow Multiple Frames in Flight #932

[cwfitzgerald]

Is this covered by #2700?

[jimblandy]

No, that doesn't have any interaction with presentation that I'm aware of. Surface texture::present doesn't return a Submission index.

Also, what (I think) we want to know is, when has a texture been presented, or perhaps, when is it done being presented? Neither of those correspond to queue activity.

[cwfitzgerald]

Superseded by 2869