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

doc/src/sgml/stylesheet-html-common.xsl

@@ -14,7 +14,7 @@
 <!-- Parameters -->
 <xsl:param name="make.valid.html" select="1"></xsl:param>
 <xsl:param name="generate.id.attributes" select="1"></xsl:param>
-<xsl:param name="ignore.image.scaling" select="1"/>
+<xsl:param name="make.graphic.viewport" select="0"/>
 <xsl:param name="link.mailto.url">pgsql-docs@lists.postgresql.org</xsl:param>
 <xsl:param name="toc.max.depth">2</xsl:param>
 

doc/src/sgml/stylesheet.css

@@ -102,3 +102,10 @@ var		{ font-family: monospace; font-style: italic; }
 acronym		{ font-style: inherit; }
 
 .option		{ white-space: nowrap; }
+
+/* make images not too wide on larger screens */
+@media (min-width: 800px) {
+  .mediaobject {
+    width: 75%;
+  }
+}