tdf#160594: Use the recommended fallback of 0.5em for ex in font-size

This fixes the error of identical treatment of em and ex in font-size,
which violated https://drafts.csswg.org/css-values-4/#font-relative-length.
The fix uses the fallback of 0.5em for ex, similar to the code used in
SvgNumber::solveNonPercentage. A follow-up should implement the correct
use of "x-height of the first available font".

Change-Id: Id9d581994e158d629d9752299ad93ac7e9fe4cad
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166234
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

svgio/qa/cppunit/SvgImportTest.cxx

@@ -1677,6 +1677,22 @@ CPPUNIT_TEST_FIXTURE(Test, testDyInEms)
     assertXPath(pDocument, "//textsimpleportion[2]"_ostr, "y"_ostr, u"32"_ustr);
 }
 
+CPPUNIT_TEST_FIXTURE(Test, testExs)
+{
+    // tdf#160594 given an SVG file with <tspan dy="3ex" style="font-size:1ex">:
+    xmlDocUniquePtr pDocument = dumpAndParseSvg(u"/svgio/qa/cppunit/data/dy_in_exs.svg");
+
+    assertXPath(pDocument, "//textsimpleportion"_ostr, 2);
+    assertXPath(pDocument, "//textsimpleportion[1]"_ostr, "height"_ostr, u"16"_ustr);
+
+    sal_Int32 nSize = getXPath(pDocument, "//textsimpleportion[2]"_ostr, "height"_ostr).toInt32();
+    // Without the accompanying fix in place, this test would have failed with:
+    // - Expected less than: 16
+    // - Actual  : 16
+    // i.e. the parent font-size was used, instead of its x-size.
+    CPPUNIT_ASSERT_LESS(sal_Int32(16), nSize);
+}
+
 CPPUNIT_PLUGIN_IMPLEMENT();
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

svgio/qa/cppunit/data/dy_in_exs.svg

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+
+<svg xmlns="http://www.w3.org/2000/svg" width="1in" height="1in" viewBox="0 0 100% 100%">
+
+	<text x="5" y="20" style="font-size:16px;font-family:Liberation Sans">foo
+		<tspan x="5" dy="3ex" style="font-size:1ex">bar</tspan></text>
+</svg>

svgio/source/svgreader/svgstyleattributes.cxx

@@ -2642,11 +2642,11 @@ namespace svgio::svgreader
                     if(pSvgStyleAttributes)
                     {
                         const SvgNumber aParentNumber = pSvgStyleAttributes->getFontSizeNumber();
+                        double n = aParentNumber.getNumber() * maFontSizeNumber.getNumber();
+                        if (SvgUnit::ex == maFontSizeNumber.getUnit())
+                            n *= 0.5; // FIXME: use "x-height of the first available font"
 
-                        return SvgNumber(
-                            aParentNumber.getNumber() * maFontSizeNumber.getNumber(),
-                            aParentNumber.getUnit(),
-                            true);
+                        return SvgNumber(n, aParentNumber.getUnit());
                     }
                 }