Better scaling of images in HTML output

Better scaling of images in HTML output

Turn on the previously disabled automatic scaling of images in HTML
output.  To avoid images looking too large on nowadays-normal screens,
restrict the width to 75% on such screens.

Some work is still necessary because SVG images without a viewBox
still won't scale, but that will a separate patch.

Discussion: https://www.postgresql.org/message-id/flat/6d2442d1-84a2-36ef-e014-b6d1ece8a139%40postgresql.org