Log DXC/FXC and shader model at startup on the DirectX12 backend

## What problem does this solve or what need does it fill?

Some GPU features (SM6+) require DXC, which is not the default DX12 shader compiler. Even if your GPU should support them, some features might be reported as not available, which is confusing.

## What solution would you like?

Log whether the app is using FXC or DXC at startup, and the max supported shader model.

## Additional context

I was trying to run my app through a debugger, and was confused why it was telling me some features were not supported. I had forgotten to copy DXC to the target directory of my app, so it was silently using FXC.