Fix middle-anchoring for lines of multiple TSpans

[mjmasn]

Summary

Fixes #1205

Not really intended as the final fix for this issue, I only dabble in Java so I'm sure there is probably a better way to do it. Happy to make changes as required :)

Test Plan

Check that lines comprising multiple <TSpan>s are centred correctly as in the 'after' screenshot:

Before	After

What's required for testing (prerequisites)?

Clone the repo here: https://github.com/mjmasn/TSpanIssue
Install react-native-svg from git branch / yarn link etc.

What are the steps to reproduce (after prerequisites)?

Run the app before and after installing this patch.

Compatibility

OS	Implemented
iOS	n/a (issue not tested on iOS)
Android	✅

Checklist

• I have tested this on a device
• and a simulator
• I added the documentation in README.md
• I mentioned this change in CHANGELOG.md
• I updated the typed files (TS and Flow)
• I added a sample use of the API in the example project (example/App.js)

[mjmasn]

Hmm, so this doesn't actually work if the spans have different fontSizes. Can't seem to work out how to get the font for each span from getSubtreeTextChunksTotalAdvance of another span.