Use Spans and ReadOnlyCollection.Empty in TextBlock/Line, reduce allocs

[h3xds1nz]

Description

Bringing some optimizations to TextBlock control and related components (Line; ComplexLine).

• Empty Initialize method in TextBlock's ctor has been removed.
• Unused private methods GetLineIndexFromDcp and ClearComplexContent have been removed (EnsureComplexContent handles the detach if its needed)
• Instead of using new ReadOnlyCollection<T>(new List<T>(0)) numerous times, we swap to the non-allocating, generic singleton variant of ReadOnlyCollection<T>.Empty, removing all possible overhead.
• Replaced ArrayList for InlineObjects in ComplexContent with List<InlineObject>.
• Unused calculated variable removed in ComplexLine - double adjustedXOffset (method doesn't modify state either)
• Unused calculated variable removed in AlignContent - Vector contentOffset (method doesn't modify state either)
• GetRangeBounds in Line instead of using a List<Rect> now uses pre-allocated Rect[] that's wrapped in ReadOnlySpan<Rect> to its callers on return.
• Instead of creating List<TextSegment>(1) and using Insert (that's a great combination), we use TextSegment[].
• I've adjusted the weird loop GetTightBoundingGeometryFromTextPositions with more thoughtful impl.

new ReadOnlyCollection<T>(new List<T>(0)) vs Empty

Method	Mean [ns]	Error [ns]	StdDev [ns]	Gen0	Code Size [B]	Allocated [B]
Original	6.0508 ns	0.1542 ns	0.2577 ns	0.0033	78 B	56 B
PR_EDIT	0.6467 ns	0.0061 ns	0.0051 ns	-	14 B	-

GetRangeBounds call old vs. new and the loop itself; without combining geometries

GetTightBoundingGeometryFromTextPositions inner loop (1 rect)

Method	Mean [ns]	Error [ns]	StdDev [ns]	Gen0	Code Size [B]	Allocated [B]
Original_string	131.0 ns	1.93 ns	1.71 ns	0.0210	1,682 B	352 B
APR_EDIT_int	112.3 ns	1.14 ns	1.07 ns	0.0134	975 B	224 B

Customer Impact

Increased performance, decreased allocations.

Regression

No.

Testing

Local build, sample apps run.

Risk

Should be low, the changes are pretty much mechanical.

Microsoft Reviewers: Open in CodeFlow