Ref: #1379
This takes a naive approach to font-width computation, the most compute-intensive part of rendering badges.
1. Add the widths of the individual characters.
- These widths are measured on startup using PDFKit.
2. For each character pair, add a kerning adjustment
- The difference between the width of each character pair, and the sum of the characters' separate widths.
- These are computed for each character pair on startup using PDFKit.
3. For a string with characters outside the printable ASCII character set, fall back to PDFKit.
This branch averaged 0.041 ms in `makeBadge`, compared to 0.144 ms on master, a speedup of 73%. That was on a test of 10,000 consecutive requests (using the `benchmark-performance.sh` script, now checked in).
The speedup applies to badges containing exclusively printable ASCII characters. It wouldn't be as dramatic on non-ASCII text. Though, we could add some frequently used non-ASCII characters to the cached set.
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