Add XSL stylesheet to fix up SVG files

The SVG output produced by external tools needs some postprocessing.
This is implemented by this new XSL stylesheet.

Issues are:

- SVG produced by Ditaa does not add a viewBox attribute to the svg
  element, needed to make the image scalable.

- SVG produced by Graphviz uses a stroke="transparent" attribute,
  which is not valid SVG.  It appears to mostly work, but FOP
  complains.

Other tweaks can be added over time.

This reverts 7dc78d8 and
29046c4, which applied these fixes
manually.

Author: petere

.gitattributes

@@ -13,7 +13,6 @@ README.*	conflict-marker-size=32
 
 # Certain data files that contain special whitespace, and other special cases
 *.data						-whitespace
-*.svg						whitespace=-blank-at-eol
 contrib/pgcrypto/sql/pgp-armor.sql		whitespace=-blank-at-eol
 src/backend/catalog/sql_features.txt		whitespace=space-before-tab,blank-at-eof,-blank-at-eol
 

doc/src/sgml/images/Makefile

@@ -8,11 +8,19 @@ ALL_IMAGES = \
 
 DITAA = ditaa
 DOT = dot
+XSLTPROC = xsltproc
 
 all: $(ALL_IMAGES)
 
-%.svg: %.gv
+%.svg.tmp: %.gv
 	$(DOT) -T svg -o $@ $<
 
-%.svg: %.txt
+%.svg.tmp: %.txt
 	$(DITAA) -E -S --svg $< $@
+
+# Post-processing for SVG files coming from other tools
+#
+# Use --novalid to avoid loading SVG DTD if a file specifies it, since
+# it might not be available locally, and we don't need it.
+%.svg: %.svg.tmp fixup-svg.xsl
+	$(XSLTPROC) --novalid -o $@ $(word 2,$^) $<

doc/src/sgml/images/README

@@ -22,6 +22,10 @@ Therefore, any tool used needs to be able to produce SVG.
 This directory contains makefile rules to build SVG from common input
 formats, using some common styling.
 
+fixup-svg.xsl applies some postprocessing to the SVG files produced by
+those external tools to address assorted issues.  See comments in
+there, and adjust and expand as necessary.
+
 Both the source and the SVG output file are committed in this
 directory.  That way, we don't need all developers to have all the
 tools installed.  While we accept that there could be some gratuitous
@@ -59,10 +63,3 @@ Notes:
 - The width should be set to something.  This ensures that the image
   is scaled to fit the page in PDF output.  (Other widths than 100%
   might be appropriate.)
-
-- SVG images should be scalable as they will be rendered in a variety
-  of places (web, PDF, etc.) as well as in different viewports
-  (desktop, mobile, etc.). To help the images successfully scale,
-  employ a "viewBox" attribute in the SVG tag. For example,
-  to create an image with a default width and height of 400x300,
-  you would use viewBox="0.00 0.00 400.00 300.00"

doc/src/sgml/images/fixup-svg.xsl

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="utf-8"?>
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+                xmlns:svg="http://www.w3.org/2000/svg"
+                version="1.0">
+
+<!--
+Transform the SVG produced by various tools, applying assorted fixups.
+-->
+
+<!--
+Add viewBox attribute to svg element if not already present.  This allows the
+image to scale.
+-->
+<xsl:template match="svg:svg">
+  <xsl:copy>
+    <xsl:if test="not(@viewBox)">
+      <xsl:attribute name="viewBox">
+        <xsl:text>0 0 </xsl:text>
+        <xsl:value-of select="@width"/>
+        <xsl:text> </xsl:text>
+        <xsl:value-of select="@height"/>
+      </xsl:attribute>
+    </xsl:if>
+    <xsl:apply-templates select="@* | node()"/>
+  </xsl:copy>
+</xsl:template>
+
+<!--
+Fix stroke="transparent" attribute, which is invalid SVG.
+-->
+<xsl:template match="@stroke[.='transparent']">
+  <xsl:attribute name="stroke">none</xsl:attribute>
+</xsl:template>
+
+<!--
+copy everything else
+-->
+<xsl:template match="@* | node()">
+  <xsl:copy>
+    <xsl:apply-templates select="@* | node()"/>
+  </xsl:copy>
+</xsl:template>
+
+</xsl:stylesheet>