update xgpu::Instance type erasure

[raustin9]

Update the type erasure implementation for the graphics API wrapper objects.

We want to make sure that wrapper objects (especially ones that might be in the rendering loop hot path) have as little overhead as possible. We also want to make working on these objects manageable.

The current idea is using a manual vtable for an interface combined with an object held in a buffer inside the wrapper rather than on the heap.

class Wrapper
{
private:
  struct VTable
  {
    std::function<auto(void*)->void> destroy;
  };
  char8_t buffer[BacknedObjectBufferSize];
};