Marshaling to shader constants buffers should not use `Unsafe.SkipInit()` to avoid garbage in padding bytes

[rickbrew]

As per discussion on Discord, https://discord.com/channels/143867839282020352/960223751599976479/1248401471813189795

The risk here is that two bitwise-equal shader structs could produce two constants buffers that are bitwise non-identical due to not setting the padding areas to zero.

This then defeats any sort of optimizations for checking to see if a property's new value is actually different than the old one, which then permits skipping ValueChanged events and also re-running PrepareForRender() and the like.

This is something I've seen significant performance wins from -- in my D2D1Properties wrapper, I check to see if setting an IUnknown* property will result in the same IUnknown* identity pointer, and then I don't forward the SetValue() call to Direct2D. This resulted in significant, measurable, even noticeable (framerate) performance improvements.

[rickbrew]

Here's the code in my ID2D1Properties wrapper where I perform my own optimization to skip the ID2D1Properties::SetValue() call, which then does not result in D2D thinking it needs to re-run PrepareForRender(), nor re-do layout (MapInputRectsToOutputRect), nor invalidate the output cache (if the Cached property is set, which I do occasionally use), etc