Improve test coverage, related bug fixes

[mike42]

Test coverage aside, some minor bug fixes are:

• PNG reader
  • Fix for low-resolution files, where a pass with 0 vertical pixels would trigger a warning
  • Validate image/bit-depth combination, to throw Exception and avoid warnings
• Include filename in error messages (when blob is loaded from file)
• Indexed images:
  • Fixes off-by-one error in quantization which caused incorrect output for low color depths (2 or 3 levels).
  • Fixes warning thrown due to null being used as the default palette, [] is now used, and expanded out to contain one color.

[coveralls]

Pull Request Test Coverage Report for Build 120

• 21 of 22 (95.45%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+53.5%) to 85.564%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Mike42/GfxPhp/IndexedRasterImage.php	1	2	50.0%

Totals
Change from base Build 112:	53.5%
Covered Lines:	1055
Relevant Lines:	1233

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💛 - Coveralls