[Highlight Pseudos] Correctly paint shadows

All highlight pseudos apart from custom highlights support text-shadow, but we were not computing overflow and we were not correctly painting or clipping. Fix it.

Existing tests cover selection alone. New tests cover highlight overlay painting, and standalone spelling, grammar and target-text.

Bug: 40276880 Change-Id: I07da6fb39a497f6ab07c8907128d87fa5554fbfc Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5480767 Commit-Queue: Stephen Chenney schenney@chromium.org Reviewed-by: Fredrik Söderquist fs@opera.com Cr-Commit-Position: refs/heads/main@{#1292045}