RFR: 8308191: [macOS] VoiceOver decorations are shifted on second monitor

[Alexander Zuev]

The coordinate system of the Mac OS X is reversed to the JavaFX coordinate system meaning that zero point [0,0] is the bottom left corner of the main screen and positive Y coordinate means elevation of the point above the zero line. Taking that into account we should use the main screen's height in order to calculate the correct vertical offset of the window - even if coordinates are physically on the secondary screen, otherwise on screens with different height that are not aligned along the bottom border we will have a wrong vertical position.

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Commit messages:
 - 8308191: [macOS] VoiceOver decorations are shifted on second monitor

Changes: https://git.openjdk.org/jfx/pull/1147/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1147&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308191
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1147.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1147/head:pull/1147

PR: https://git.openjdk.org/jfx/pull/1147