RFR: 8298167: Opacity in WebView not working anymore [v4]

[Roman Marchenko]

> There was refactoring in WebKit's GraphicsContext class (nativecode) https://github.com/WebKit/WebKit/commit/1733b8bc3dff7595ab8e42561fc0f20a2b8fee63 
> 
> GraphicsContextJava's methods begin/endPlatformTransparencyLayers weren't adapted after GraphicsContext's refactoring integration into JFX. As the result, the methods were not invoked, so TransparencyLayer couldn't be added to a rendering queue.
> 
> Now the methods are fixed to be used properly in GraphicsContext inheritance chain.

Roman Marchenko has updated the pull request incrementally with one additional commit since the last revision:

  OpacityTest checks pixel color at the centre of areas

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Changes:
  - all: https://git.openjdk.org/jfx/pull/992/files
  - new: https://git.openjdk.org/jfx/pull/992/files/a349829c..e236741f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=992&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=992&range=02-03

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/992.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/992/head:pull/992

PR: https://git.openjdk.org/jfx/pull/992