[OpenJDK 2D-Dev] RFR: 8269223: -Xcheck:jni WARNINGs working with fonts on Linux [v6]

[Maxim Kartashev]

> Added an `ExceptionCheck()` followed by `ExceptionDescribe()` and `ExceptionClear()` immediately after the Java calls made from the callback function `ReadTTFontFileFunc()` in `freetypeScaler.c`. 
> 
> The exception(s) need to be cleared because we're not returning immediately to Java that would've been able to handle them gracefully. And in order not to loose the exception entirely (even though the return value would also indicate an error condition), print out the exception with `ExceptionDescribe()` to aid in debugging.

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

  Address PR comments
  
  1. Added code to log the exception if it occurred when running a flushBuffer's runnable. The logging is minimal in order to avoid replacing the exception with another one before it reaches Java.
  
  2. Made the test not dependent on headful code as per @prrace suggestions.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4572/files
  - new: https://git.openjdk.java.net/jdk/pull/4572/files/13b08686..4b2febee

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4572&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4572&range=04-05

  Stats: 9 lines in 2 files changed: 4 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4572.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4572/head:pull/4572

PR: https://git.openjdk.java.net/jdk/pull/4572