RFR: 8364434: Inconsistent BufferedContext state after GC [v2]

[Nikita Gubarkov]

> For "true" null objects, reset the ref itself to null. Non-null ref with null content means that the object was GC'ed. GC'ed state always behaves as not-equal to the new one, causing corresponding ops to be written into RQ.
> 
> Although I could not find practical scenarios where refs other than `validPaintRef` could cause problems, this is generally fragile and potentially problematic for any state object kept in weak ref. Therefore I changed the usage of all weak refs in the same way.

Nikita Gubarkov has updated the pull request incrementally with one additional commit since the last revision:

  8364434: Inconsistent BufferedContext state after GC
  
  Add test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26576/files
  - new: https://git.openjdk.org/jdk/pull/26576/files/849c74ce..822dc899

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26576&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26576&range=00-01

  Stats: 77 lines in 1 file changed: 77 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26576.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26576/head:pull/26576

PR: https://git.openjdk.org/jdk/pull/26576