Make sure setting a render equal picture does not result in paint updates

[dnfield]

This is basically #655 with a test, and also bumping some changelog/pubspec stuff.

[coveralls]

Pull Request Test Coverage Report for Build 5886909158260736

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 52 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-2.4%) to 84.218%

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Files with Coverage Reduction	New Missed Lines	%
lib/src/render_picture.dart	1	82.35%
lib/src/utilities/numbers.dart	1	88.89%
lib/src/picture_cache.dart	3	88.24%
lib/src/svg/parser_state.dart	3	94.61%
lib/src/picture_stream.dart	4	60.2%
lib/svg.dart	4	81.44%
lib/src/picture_provider.dart	10	51.79%
lib/src/vector_drawable.dart	26	86.74%

Totals
Change from base Build 5672608678019072:	-2.4%
Covered Lines:	1745
Relevant Lines:	2072

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💛 - Coveralls