[set] Change pagesize from 512 bits to 8192 bits

Fixes perf regression on some heavy fonts in Chrome's FT+HB
interaction.

See:
https://bugs.chromium.org/p/chromium/issues/detail?id=782220

More work to be done:
https://bugs.chromium.org/p/chromium/issues/detail?id=794896
1 file changed