Implement Rect as position and size vectors

[kimci86]

This groups sf::Rect members left, top, width, height into two vectors: position and size.

This makes sf::Rect more aligned with all SFML API working with Vector2 instead of separate values.
Related: #2055, #2248

[coveralls]

Pull Request Test Coverage Report for Build 8990773841

Details

• 104 of 109 (95.41%) changed or added relevant lines in 11 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-4.8%) to 51.903%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/SFML/Graphics/RenderTarget.cpp	9	11	81.82%
src/SFML/Graphics/Font.cpp	16	19	84.21%

Files with Coverage Reduction	New Missed Lines	%
src/SFML/Graphics/Text.cpp	2	86.46%

Totals
Change from base Build 8989395986:	-4.8%
Covered Lines:	10428
Relevant Lines:	18957

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💛 - Coveralls

[vittorioromeo]

Not sure how I feel about this yet, just a few random comments after a first skim.

[ChrisThrasher]

I really like this idea. Conceptually a Rect has always been a position and a size. For a long time we've agreed it's better to represent positions and sizes as vectors since that is more expressive and lends itself to better composition with other APIs that return and accept vectors for positions and sizes. I see no reason that Rect ought to be recused from that same precedent.

Any ergonomic issues introduced by this refactor can be fixed by adding the right new member functions.

[kimci86]

Note that I'm keeping separate commits for the review but I'll squash related commits once we are ready to merge.