RFR: JDK-8297449: Update JInternalFrame Metal Border code [v2]

[Harshitha Onkar]

> Updated Metal Border code for JInternalFrame. 
> 
> - Added instanceof check before casting Graphics to G2D.
> - Replaced roundHalfDown with Region.clipRound()

Harshitha Onkar has updated the pull request incrementally with one additional commit since the last revision:

  review changes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11305/files
  - new: https://git.openjdk.org/jdk/pull/11305/files/0e7e7e7d..5f019dfd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11305&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11305&range=00-01

  Stats: 121 lines in 1 file changed: 40 ins; 34 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/11305.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11305/head:pull/11305

PR: https://git.openjdk.org/jdk/pull/11305