WIP | Implement background-size contain and cover

[locks]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 709

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 202 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 0.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
azul-css/src/css_properties.rs	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
target/debug/build/azul_glium-2fd4acd17fff9eef/out/gl_bindings.rs	202	0.0%

Totals
Change from base Build 707:	-0.07%
Covered Lines:	0
Relevant Lines:	1420

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💛 - Coveralls

[fschutt]

Yeah, I don't have anything to review, this is more or less how it should be. One could argue that it would be better to use &Option<StyleBackgroundSize>, instead of Option<StyleBackgroundSize>, but it probably doesn't matter. Thanks for taking the time to do this.