Fix rounding in SSE byte stores

[Nathan Chandrasekar (vera-rubin)]

On SSE, storing XMVectorReplicate(0.75f) with XMStoreByte4 or XMStoreUByte4 produces 0x00000001 instead of 0x01010101. XMStoreByteN4 similarly packs 0.25f as 0x1f1f1f1f instead of 0x20202020. The documented Byte4, UByte4, and ByteN4 operations round each component before packing.

We convert each clamped component to an integer before packing with SSE2 instructions, following the short-store implementation.

We checked the reproducer on SSE2, SSE4.1, and AVX2 and added a math3 regression that fails before and passes after, while the full suite retains its five existing math3 failures.

— ChipLab

[Chuck Walbourn (walbourn)]

I'll need to look closely at these proposed changes. There are a number of issues where I need to ensure that ARM matches, and that it doesn't negatively impact DirectXTex, etc. which is the primary consumer of these functions.