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

[SWinxy]

On Mon, 28 Nov 2022 20:25:19 GMT, Harshitha Onkar <honkar at openjdk.org> wrote:

>> 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

src/java.desktop/share/classes/javax/swing/plaf/metal/MetalBorders.java line 267:

> 265:             }
> 266: 
> 267:             AffineTransform at = null;

Because we'd no longer be causing a `ClassCastException` from casting this to be a `Graphics2D` object, later down on line 311, if the `Graphics` object isn't a G2D type, then it will throw an NPE. I fear matrices, so I have no idea what the value of `getScaleX()` would be otherwise--replace it with zero if null?

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