Only poll device when reading data instead of blocking

[ArthurBrussee]

Changes

According to wgpu docs, the device only needs to be polled for the map_async to complete at some point. Waiting for the channel does the actual waiting (async or blocking).

This is not very different, but, Maintain::Wait also prevents other threads from submitting commands, leading to stalls in multithreaded scenarios. Here's a before and after of a multithreaded app:

(Send batch is waiting on a read).

[ArthurBrussee]

Hm, converting this to draft for a sec as it seems tests are timing out waiting for data :/ Maybe poll isn't enough after all, will investigate.