minipage should update \textwidth, \columnwidth

[xworld21]

Discovered while checking the tests figure_mixed_content, figure_dual_caption. Figures have the wrong size because they use \textwidth, which isn't adjusted inside minipage.

Per LaTeX2e sources, minipage updates \textwidth, \columnwidth within a non-boxing group.

[dginev]

A fix here is likely to improve:

• figure 1 is much smaller in the pdf arXiv/html_feedback#2679
• Table's Pictures Error arXiv/html_feedback#2709