RFR: 8313697: [XWayland][Screencast] consequent getPixelColor calls are slow

[Alexander Zvegintsev]

Right now, each call to getPixelColor/createScreenCapture opens and closes a ScreenCast session. 
When there are many such calls in a row, it becomes a rather time-consuming operation.

For example,


for (int i = 0; i < 50; i++) {
    for (int j = 0; j < 50; j++) {
        robot.getPixelColor(i, j);
    }
} 

`50x50 took 375219ms`

This fix offers optimization by keeping the session open and closing it only 2 seconds after the last activity.

`50x50 took 28113ms` which is ~ 13 times faster.

Testing looks good.

-------------

Commit messages:
 - Handle the case where a new screen capture requires a token different from the one used by the current session.
 - initial

Changes: https://git.openjdk.org/jdk/pull/15250/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15250&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313697
  Stats: 161 lines in 7 files changed: 111 ins; 23 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/15250.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15250/head:pull/15250

PR: https://git.openjdk.org/jdk/pull/15250