Include sizeof(T) in offset calculation for Memory2D from MemoryManager

[Lillenne]

Closes #742

Overview

Memory2D<T> and ReadOnlyMemory2D<T> instances created from a MemoryManager calculate the offset field in units of T instead of byte in the constructor. Since this fix in v8.2.1 where Span creation was changed to Unsafe.AddByteOffset, this causes incorrect indexing into the span.

Fixes were added for NET6 or greater.

PR Checklist

• [X] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [X] Based off latest main branch of toolkit
• [X] PR doesn't include merge commits (always rebase on top of our main, if needed)
• [X] Tested code with current supported SDKs
• [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [X] Contains NO breaking changes
• [X] Code follows all style conventions

[Lillenne]

@dotnet-policy-service agree

[Lillenne]

@dotnet-policy-service agree