Add exposed function to retrieve all unicode codepoints defined in fonts

[CptWesley]

Prerequisites

• [X] I have written a descriptive pull-request title
• [X] I have verified that there are no overlapping pull-requests open
• [X] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [X] I have provided test coverage for my change (where applicable)

Description

Fixes #307

As described in the suggestion, exposing this information can make it easier for users to retrieve some extra information about what code points and glyphs are available in a font and how they are being mapped. Whether or not this is the right strategy to allow these queries is of course up for debate.

Changes:

• Added GetAvailableCodePoints function to Font type which retrieves all defined code points in that particular font.
• Made GlyphId field in GlyphMetrics public.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #308 (51afd7c) into main (14b89e6) will decrease coverage by 1%. The diff coverage is 80%.

@@          Coverage Diff          @@
##            main    #308   +/-   ##
=====================================
- Coverage     83%     83%   -1%     
=====================================
  Files        224     224           
  Lines      12311   12331   +20     
  Branches    1787    1790    +3     
=====================================
+ Hits       10290   10306   +16     
- Misses      1601    1603    +2     
- Partials     420     422    +2     

Flag	Coverage Δ
unittests	83% <80%> (-1%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/SixLabors.Fonts/FontMetrics.cs	100% <ø> (ø)
...ixLabors.Fonts/Tables/General/CMap/CMapSubTable.cs	90% <ø> (ø)
...bors.Fonts/Tables/General/CMap/Format14SubTable.cs	78% <0%> (-2%)	:arrow_down:
src/SixLabors.Fonts/Tables/General/CMapTable.cs	85% <66%> (-3%)	:arrow_down:
src/SixLabors.Fonts/StreamFontMetrics.cs	87% <75%> (-1%)	:arrow_down:
src/SixLabors.Fonts/FileFontMetrics.cs	96% <100%> (+<1%)	:arrow_up:
src/SixLabors.Fonts/GlyphMetrics.cs	50% <100%> (ø)
...abors.Fonts/Tables/General/CMap/Format0SubTable.cs	100% <100%> (ø)
...bors.Fonts/Tables/General/CMap/Format12SubTable.cs	97% <100%> (+<1%)	:arrow_up:
...abors.Fonts/Tables/General/CMap/Format4SubTable.cs	100% <100%> (ø)

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[JimBobSquarePants]

I haven't forgotten about this. I'm just doing some research to ensure we can implement the correct thing.

[CptWesley]

@JimBobSquarePants My apologies for no longer replying due to the holiday season. I have not incorporated the requested changes due to the fact that it seems this might not be the right approach for solving this due to what you mentioned and what was mentioned in #307. My personal experience with font rendering is very minimal, so I'm not sure what the right approach would be, but if I can be of any help, please let me know.

[JimBobSquarePants]

Hi @CptWesley I did some research, and I cannot find a definitive answer regarding variations. As such I believe it's best to leave as-is.

The rest of your code is ace. All I changed was the review comments re optimization. Thanks!

[CptWesley]

@JimBobSquarePants Thank you very much!