Support recording swapchain commands from multiple threads

[kvark]

We had a nasty piece here:

// This is totally unsafe: if multiple threads would start recording render passes into

// that uses this swapchain, we'd have a race condition.

It was a shortcut to get things going when the new swapchain model was implement. However, it appears that applications do need that!

The solution involves fancy parking_lot mutex mapping with custom derefs, and it works great:

[kvark]

bors r+

[bors[bot]]

Timed out.