fix: Enhance font weight determination for variable fonts

[gadfly3173]

What does the pull request do?

Fixes #18875 , enhance font weight determination for variable fonts.

What is the current behavior?

Font weight of the variable fonts is not correct.

What is the updated/expected behavior with this PR?

    <!-- Cascadia Code font -->
    <TextBlock Text="This is normal text in Cascadia Code font"
               FontFamily="Cascadia Code"
               FontSize="16" />
    <TextBlock Text="This is bold text in Cascadia Code font"
               FontFamily="Cascadia Code"
               FontWeight="Bold"
               FontSize="16" />

    <!-- Noto Sans font -->
    <TextBlock Text="This is normal text in Noto Sans font"
               FontFamily="Noto Sans SC"
               FontSize="16" />
    <TextBlock Text="This is bold text in Noto Sans font"
               FontFamily="Noto Sans SC"
               FontWeight="Bold"
               FontSize="16" />

    <!-- Dejavu Sans Mono font -->
    <TextBlock Text="This is normal text in Dejavu Sans Mono font"
               FontFamily="Dejavu Sans Mono"
               FontSize="16" />
    <TextBlock Text="This is bold text in Dejavu Sans Mono font"
               FontFamily="Dejavu Sans Mono"
               FontWeight="Bold"
               FontSize="16" />

    <!-- 思源黑体 CN font -->
    <TextBlock Text="This is normal text in 思源黑体 CN font"
               FontFamily="思源黑体 CN"
               FontSize="16" />
    <TextBlock Text="This is bold text in 思源黑体 CN font"
               FontFamily="思源黑体 CN"
               FontWeight="Bold"
               FontSize="16" />

    <!-- Roboto Flex font -->
    <TextBlock Text="This is normal text in Roboto Flex font"
               FontFamily="Roboto Flex"
               FontSize="16" />
    <TextBlock Text="This is bold text in Roboto Flex font"
               FontFamily="Roboto Flex"
               FontWeight="Bold"
               FontSize="16" />
  </StackPanel>

How was the solution implemented (if it's not obvious)?

Added variable font detection via 'fvar' table check

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes #18875

[rabbitism]

Could you please also help to check if this is fixed? #18677

[cla-avalonia]

• All contributors have signed the CLA.

[gadfly3173]

@cla-avalonia agree

[gadfly3173]

Could you please also help to check if this is fixed? #18677

It's not fixed. I don't think your issue is related to the font weight determination.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0056617-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0056619-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Gillibald]

I can't accept this hack. We need to add proper variable fonts support and actually control the wght axis.

Variable fonts are not supported at the moment. Looking for the presence of fvar isn't enough.

[gadfly3173]

Looking for the presence of fvar isn't enough.

I agree. SKTypeface lacks sufficient information to determine if it’s a variable font. However, variable font rendering works correctly below version 11.3. Should we revert the Weight assignment to the following logic?

Weight = (fontSimulations & FontSimulations.Bold) != 0 ? FontWeight.Bold : (FontWeight)typeface.FontWeight;

宋体 in the following pic is a single-weight, non-italic typeface. Its rendering effect is identical to #18113, while also supporting variable fonts.

#18113 effect:

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0056621-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]