Bugfixes:

• SVG: don't convert polyline/rect/polygon/line to path, which can break CSS, fixes #260
• SVG: relative SVG Bézier commands not properly minified
• CSS: don't remove whitespace in nested unknown at-rules, fixes #262
• CSS: fix panic for background when it contains functions other than calc for background-position, fixes #263
• CSS: fix panic for background-position with three numbers
• CSS: fix panic for url() with only whitespace or only one quote