Important internal optimizations, potential bug

[calebwin]

There are several internal things that make Emu's performance potentially suboptimal. This issue is a place to discuss them.

• wgpu::Device::poll is used here and right now it blocks in an async context. I'm not sure what the solution is but there is some discussion here.
• right now, we use a different staging buffer for every DeviceBox. This should be replaced by some sort of global pool of staging buffers which is shared by all DeviceBoxs.

Edit: the Device::poll thing is probably not an optimization but a bug in the library