Transform the empty backend into a mock

[kvark]

Doing nothing is often better than panicking. For example, it would allow us to test the internal logic of gfx-memory: we could randomly spam allocations and check for consistency of the allocator internal state.

[GabrielMajeri]

Alright, this issue is now partially solved and we've got fuzz tests for gfx-memory.

What would be the next step? Mocking the descriptor pool API to fuzz gfx-descriptor?

[kvark]

Yes, that would be very useful as well!