Make line breaks cause full-width box when they are at the top level. (#1804)

This PR makes HTML-CSS, CHTML, and SVG output produce the same results for when \\ is used outside of arrays and alignments. If the \\ is at the top level, it will produce a full-width box (so that if used in-line, you will not get strange artifacts due to the width being smaller than the line width), but if used internally, it will have a tight bounding box. So text $a\\b$ text should produce

text
a
b
text

rather than

text a text
     b

or

text
a text
b

as currently happens in SVG and HTML-CSS output.

Resolves issue #1804 (closed) and #1802 (closed).