Use gfx-rs' pipeline cache object

[zakorgy]

Currently creating a new Renderer instance in Gecko or resizing the window takes too much time, especially with DX12. We can speed up this by using pipeline cache from gfx-rs.
Pipeline cache support in gfx-rs on different platforms:

• DX12
• Vulkan
• Metal

Adding the code for cache creation/usage on the WR side would be an easy task. What we have to figure out is how to store the cache between multiple runs. Unfortunately there is no functionality in gfx-rs, to write\load a pipeline cache into/from a file yet. As a workaround we could try to store/update the cache in the memory on the Gecko side after a Renderer instance is destroyed and use it for the next ones.

[zakorgy]

I checked this and implemented the missing parts in ash and my gfx-rs fork to enable the read/write of the pipeline caches from/into a file (Vulkan only). Also created a WIP branch where we use pipeline cache in our WR.