Optimize ComputerInkBoundingBox(LtoR), remove unnecessary additional branch

[h3xds1nz]

Description

Optimizes ComputerInkBoundingBox / ComputerInkBoundingBoxLtoR methods, collapsing additional branch for TextFormattingMode.Display into EmGlyphMetrics, which now takes a readonly ref of GlyphMetrics to avoid copies.

This is one of the hottest methods traditionally when working with glyphs, so it deserves a bit of special care. EmGlyphMetrics is just a data container with a bit of logic, and it is kind of imperative that the logic in the ctor is actually inlined inside the original method and the struct ceases to exist at runtime, otherwise due to the nature of the code, this could get pretty messy at the given JIT state.

This gets inlined usually even without forcing it via AggressiveInlining but it doesn't hurt to force it in this case, it would hurt in case it didn't get inlined, so we want to prevent that from happening.

For 8 glyphs, the difference is already significant:

Method	otherVal	Mean [ns]	Error [ns]	StdDev [ns]	Code Size [B]	Allocated [B]
Original	GlyphMetrics[8]	42.41 ns	0.251 ns	0.222 ns	2,882 B	-
PR__EDIT	GlyphMetrics[8]	34.01 ns	0.188 ns	0.166 ns	2,736 B	-

Customer Impact

Improved performance.

Regression

No.

Testing

Local build, perf benchmarks.

Risk

Imho it is low, we don't change any existing logic.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 41 lines in your changes missing coverage. Please review.

Project coverage is 11.23166%. Comparing base (b6914d7) to head (4703b5d).
Report is 5 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10630         +/-   ##
===================================================
- Coverage   11.45872%   11.23166%   -0.22706%     
===================================================
  Files           3214        3353        +139     
  Lines         648458      667987      +19529     
  Branches       71511       74978       +3467     
===================================================
+ Hits           74305       75026        +721     
- Misses        572989      591798      +18809     
+ Partials        1164        1163          -1     

Flag	Coverage Δ
Debug	11.23166% <0.00000%> (-0.22706%)	⬇️

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