RFR: 8346227: Seal Paint and Material [v2]

[Michael Strauß]

> The `Paint` and `Material` classes can't be extended by user code, because their implementations require special support in internal JavaFX code. The classes should be sealed.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  use exhaustive switch pattern

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1665/files
  - new: https://git.openjdk.org/jfx/pull/1665/files/5c4b42c8..f0a39b16

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1665&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1665&range=00-01

  Stats: 18 lines in 2 files changed: 0 ins; 12 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1665.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1665/head:pull/1665

PR: https://git.openjdk.org/jfx/pull/1665