RFR: 8316206: Test StretchedFontTest.java fails for Baekmuk font [v2]

[Alexey Ivanov]

> **Root cause**
> 
> The _Baekmuk Headline_ font maps `\u6f22` (漢) to glyph id 16950 which has zero length.
> 
> The test fails if the right half of the image with the rendered glyph contains only pixels of background colour. In this case, the left half of the image is also blank. It's somewhat false positive because of the broken font.
> 
> **Fix**
> 
> Ignore fonts which don't render the glyph correctly. If the left half of the image contains pixels of the background colour only, it is expected that the right half also contains background-coloured pixels only.
> 
> The updated test does not fail with the _Baekmuk Headline_ font. It still detects the original problem and fails on builds without the fix for [JDK-8312555](https://bugs.openjdk.org/browse/JDK-8312555).

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

  8316206: Use GlyphVector.getVisualBounds().isEmpty()
  
  Filter out broken fonts by GlyphVector.getVisualBounds().isEmpty()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15818/files
  - new: https://git.openjdk.org/jdk/pull/15818/files/6c866aea..0f8b78f4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15818&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15818&range=00-01

  Stats: 65 lines in 1 file changed: 26 ins; 32 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15818.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15818/head:pull/15818

PR: https://git.openjdk.org/jdk/pull/15818