RFR: 8273946: Move clearQuad method to BaseShaderGraphics superclass

[Jose Pereda]

On Tue, 21 Sep 2021 12:45:20 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> As mentioned in JBS, the `clearQuad` methods in `D3DGraphics` and `ES2Graphics` are now identical, which should have been the case all along. The fact that they weren't identical was the source of a bug that was only discovered during the testing of [JDK-8090547](https://bugs.openjdk.java.net/browse/JDK-8090547) on the es2 pipeline.
> 
> This PR moves the `clearQuad` method to the `BaseShaderGraphics` superclass to get rid of the unnecessary code duplication. This will be helpful when implementing the metal pipeline as well. As a note, I made the `context` field in the `D3DGraphics` final, which it should have been. This is necessary to ensure that moving the method to the super-class is equivalent.
> 
> No new tests are needed, since this is just refactoring.

Looks good to me

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Marked as reviewed by jpereda (Committer).

PR: https://git.openjdk.java.net/jfx/pull/628