RFR: 8017266: Background is painted taller than needed for styled text.

[Prasanta Sadhukhan]

GlyphView.paint() draws background bounding the passed Shape, while the span reserved for the superscripted text is taller than the height of the glyphs so it is better to use the painter.getHeight() instead of alloc.height to fill the actual glyphs boundary

Before fix
<img width="103" height="93" alt="image" src="https://github.com/user-attachments/assets/e673d214-13c7-4a42-8f55-cfd654e5a978" />


With fix
<img width="110" height="86" alt="image" src="https://github.com/user-attachments/assets/c13377a6-b4ac-406c-a153-a55c0b9a917b" />

No regressions is observed in CI..A manual verification test is provided..

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Commit messages:
 - 8017266: Background is painted taller than needed for styled text.

Changes: https://git.openjdk.org/jdk/pull/27947/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27947&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8017266
  Stats: 81 lines in 2 files changed: 79 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27947.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27947/head:pull/27947

PR: https://git.openjdk.org/jdk/pull/27947