Rectify latest cairo SVG export

It seems Cairo got updated and its SVG export is now broken in a new and
exciting way. Now it exports documents with dimensionless w/h. The issue
with that is that Cairo thinks that means point @ 72 pt / inch, but
everyone else including the actual svg spec interpret that as raw document
units or pixels, AFAICT completely breaking dpi scaling in common
viewers. Since we have to mess with Cairo's SVG files anyway, we just
fix this as well by re-writing the broken w/h into physical units
according to a hardcoded conversion factor that matches cairo's
hardcoded scale.

gerbonara/tests/image_support.py

@@ -151,6 +151,8 @@ def svg_soup(filename):
         f.write(str(soup))
 
 def cleanup_gerbv_svg(soup):
+    soup.svg['width'] = f'{float(soup.svg["width"])/72*25.4:.4f}mm'
+    soup.svg['height'] = f'{float(soup.svg["height"])/72*25.4:.4f}mm'
     for group in soup.find_all('g'):
         # gerbv uses Cairo's SVG canvas. Cairo's SVG canvas is kind of broken. It has no support for unit
         # handling at all, which means the output files just end up being in pixels at 72 dpi. Further, it
@@ -186,15 +188,28 @@ def gerber_difference_merge(ref1, ref2, actual, diff_out=None, composite_out=Non
         gerbv_export(ref1, ref1_svg.name, size=size, export_format='svg')
         gerbv_export(ref2, ref2_svg.name, size=size, export_format='svg')
         gerbv_export(actual, act_svg.name, size=size, export_format='svg')
+        for var in ['ref1_svg', 'ref2_svg', 'act_svg']:
+            print(f'=== {var} ===')
+            print(Path(locals()[var].name).read_text().splitlines()[1])
 
         with svg_soup(ref1_svg.name) as soup1:
             if svg_transform1 is not None:
-                soup1.find('g', attrs={'id': 'surface1'})['transform'] = svg_transform1
+                svg = soup1.svg
+                children = list(svg.children)
+                g = soup1.new_tag('g', attrs={'transform': svg_transform1})
+                for c in children:
+                    g.append(c.extract())
+                svg.append(g)
             cleanup_gerbv_svg(soup1)
 
             with svg_soup(ref2_svg.name) as soup2:
                 if svg_transform2 is not None:
-                    soup2.find('g', attrs={'id': 'surface1'})['transform'] = svg_transform2
+                    svg = soup2.svg
+                    children = list(svg.children)
+                    g = soup2.new_tag('g', attrs={'transform': svg_transform2})
+                    for c in children:
+                        g.append(c.extract())
+                    svg.append(g)
                 cleanup_gerbv_svg(soup2)
 
                 defs1 = soup1.find('defs')

gerbonara/tests/test_rs274x.py

@@ -473,6 +473,8 @@ def test_svg_export(reference, tmpfile):
     ref_svg = tmpfile('Reference export', '.svg')
     ref_png = tmpfile('Reference render', '.png')
     gerbv_export(reference, ref_svg, origin=bounds[0], size=bounds[1], fg='#000000', bg='#ffffff')
+    with svg_soup(ref_svg) as soup:
+        cleanup_gerbv_svg(soup)
     svg_to_png(ref_svg, ref_png, dpi=300, bg='white')
 
     out_png = tmpfile('Output render', '.png')