Add [SuppressGCTransition] to D3D12 command recording methods

[John Kelly]

Description (optional)

Introduce [SuppressGCTransition] to calls that record commands on a command list. This does not include Close/Reset commands, nor any commands from parent types (e.g Get/SetPrivateData.

Rationale

These are some of the hottest methods in the API (this is why they don't return error codes etc). Constant GC transitions while calling them are liable to have a non-negligible impact on performance.

Drawbacks

Naturally, SuppressGCTransition has risks. In my experience, all the methods on these list have trivial execution times (sub 1-us), and they are explicitly single threaded, so they should not be taking any syncprims.

Other thoughts

An interesting alternative is some sort of method to "batch" the transitions via an API. E.g instead of what is implicitly

EnterPreemptive();
list->IASetVertexBuffers(...);
ExitPreemptive();

EnterPreemptive();
list->OMSetDepthBounds(...);
ExitPreemptive();

EnterPreemptive();
list->Draw(...);
ExitPreemptive();

being able to explicitly do

EnterPreemptive();
 
list->IASetVertexBuffers(...);
list->OMSetDepthBounds(...);
list->Draw(...);

ExitPreemptive();

[Tanner Gooding]

This will only be possible for DirectX in .NET 6+ as that is when CallConvSuppressGCTransition will be available.

I'm generally fine with doing this for Vulkan, but it would be good to understand what differences in behavior may exist for various debug layers or other features that can be enabled.

[Tanner Gooding]

This isn't feasible to do due to the debug and other layers. It will cause deadlocks and other problematic behavior.