Don't round the widths and heights of content.

Problem
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Rounding the width, height, and left positions causes misplacement by several pixels when the content is scaled.

Solution
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Remove rounding of dimensions and positioning of placeholders and content.

Testing
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All unit tests should continue to pass.
Open the ScrollList demo, everything should work normally at various scales and viewport dimensions.

@timmccall-wf @todbachman-wf @georgelesica-wf @patkujawa-wf @robbecker-wf @tomconnell-wf