docs: html: copy images to output as part of xslt build

Until now the meson built HTML docs had non-working references to images. They
were copied in the make target. Instead of duplicating that for meson, copy
them as part of the xslt stylesheet.

Reported-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/3fc3bb9b-f7f8-d442-35c1-ec82280c564a@enterprisedb.com

doc/src/sgml/Makefile

@@ -144,7 +144,6 @@ html: html-stamp
 
 html-stamp: stylesheet.xsl postgres-full.xml $(ALL_IMAGES)
 	$(XSLTPROC) $(XMLINCLUDE) $(XSLTPROCFLAGS) $(XSLTPROC_HTML_FLAGS) $(wordlist 1,2,$^)
-	cp $(ALL_IMAGES) html/
 	cp $(srcdir)/stylesheet.css html/
 	touch $@
 

doc/src/sgml/stylesheet-common.xsl

@@ -101,4 +101,26 @@
   <xsl:apply-templates select="." mode="xref"/>
 </xsl:template>
 
+
+<!--
+  Support for copying images to the output directory, so the output is self
+  contained.
+-->
+<xsl:template name="write-image">
+ <xsl:variable name="input_filename">
+   <xsl:value-of select="imagedata/@fileref"/>
+ </xsl:variable>
+
+ <!-- references images directly, without images/ -->
+ <xsl:variable name="output_filename">
+   <xsl:value-of select="concat($chunk.base.dir, substring-after($input_filename, '/'))"/>
+ </xsl:variable>
+
+ <xsl:call-template name="write.chunk">
+  <xsl:with-param name="filename" select="$output_filename"/>
+  <xsl:with-param name="content" select="document($input_filename)"/>
+ </xsl:call-template>
+</xsl:template>
+
+
 </xsl:stylesheet>

doc/src/sgml/stylesheet.xsl

@@ -29,6 +29,12 @@
 </xsl:param>
 
 
+<!-- copy images to the output directory, so the output is self contained -->
+<xsl:template match="imageobject">
+  <xsl:call-template name="write-image"/>
+  <xsl:apply-templates select="imagedata"/>
+</xsl:template>
+
 <!-- strip directory name from image filerefs -->
 <xsl:template match="imagedata/@fileref">
  <xsl:value-of select="substring-after(., '/')"/>