RFR: 8264999: GeneralPath.lineTo() to itself produces jagged lines [v3]

[Jeremy]

> Previously when you drew a line that moved exactly (+0, +0) with a miter stroke the Stroker class acted like a horizontal miter was appropriate. Now we abort instead, so no miter is applied.
> 
> The new unit test tests the original steps plus several variations.
> 
> I'm a little nervous about this change because it looks like someone previously considered this exact condition and implemented a different behavior. I don't think I understand the original rationale.

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

  JDK-8264999: GeneralPath.lineTo() to itself produces jagged lines
  
  Separating "prev == DRAWING_OP_TO" condition per Laurent's code review.
  
  I also reorganized & commented on the unit test scenarios, including a few new commented out scenarios.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/8943/files
  - new: https://git.openjdk.org/jdk/pull/8943/files/c7e35275..f14b5598

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8943&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8943&range=01-02

  Stats: 42 lines in 2 files changed: 32 ins; 8 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/8943.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/8943/head:pull/8943

PR: https://git.openjdk.org/jdk/pull/8943