Blazor Server RenderBatchWriter should stop defensive copies of RenderTreeFrame on Write method

[SteveSandersonMS]

In .NET 5, RenderTreeFrame stopped being readonly in order to support some performance optimizations focused on WebAssembly. Generally this did not have any drawbacks because:

• The mutability is internal. We didn't make it publicly mutable.
• The framework always passes around RenderTreeFrame either with ref or with no modifier
• Or in the case of RenderTreeDiffBuilder.NextSiblingIndex, it does use in but it doesn't make any method calls or property accesses inside it (we probably should change this to use ref to have the behavior be more obvious).

However there remains one other case: RenderBatchWriter's Write method:

void Write(in RenderTreeFrame frame)
{
    ....
}

Inside there, it performs lots of property reads. If this blog post about perf implications of in is correct, that means there will be lots of defensive copies happening.

To fix this, we could either remove the in modifier and just have a single copy made for each call to Write, or we could replace in with ref and have no copies made.

I don't yet have any specific measurement of the perf effects. We should at least try to measure this before committing to a specific resolution. It might not really have any observable real-world effects because RenderBatchWriter is so fast anyway relative to other things that are going on, such as transmitting render batches over the network.

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