RFR: 8304041: [WebView] Text on the left and right of emoji is not visible

[Phil Race]

This fixes a problem with mixing Emoji and regular text in Webview.
The Prism implementation expects that Emoji will not be in the same call to drawString as regular glyphs,
since they need very different handling.
This fix breaks up the runs webview hands to Prism.
The test program in the bug report now renders properly (which it never did before in any release)

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Commit messages:
 - 8304041

Changes: https://git.openjdk.org/jfx/pull/1083/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1083&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304041
  Stats: 55 lines in 1 file changed: 54 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1083.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1083/head:pull/1083

PR: https://git.openjdk.org/jfx/pull/1083