Add fixedsize as third image_size value

When True, enclose the image cell in a table without borders to avoid
narrow borders when the fixed width is less than the node width.

src/wireviz/wv_helper.py

@@ -58,8 +58,17 @@ def nested_html_table(rows):
 def html_image(node):
     if not node.image:
         return None
-    return f'''<tdX{' sides="TLR"' if node.caption else ''}{html_size_attr(node.image_size)}>''' \
-           f'''<img scale="{node.image_scale}" src="{node.image}"/>'''
+    size = html_size_attr(node.image_size)
+    # The leading attributes belong to the preceeding tag. See where used below.
+    html = f'{"".join(size)}><img scale="{node.image_scale}" src="{node.image}"/>'
+    if size[2]: # fixedsize
+        # Close the preceeding tag and enclose the image cell in a table without
+        # borders to avoid narrow borders when the fixed width < the node width.
+        html = f'''>
+<table border="0" cellspacing="0" cellborder="0"><tr>
+<td{html}</td>
+</tr></table>'''
+    return f'''<tdX{' sides="TLR"' if node.caption else ''}{html}'''
 
 def html_caption(node):
     return f'''<tdX{' sides="LRB"' if node.image else ''}>{html_line_breaks(node.caption)}''' if node.caption else None
@@ -69,10 +78,13 @@ def html_size_attr(size):
     # size: List of values where only these are used:
     # - First value is minimum width of object in points, an int value in the range 1-65535. (Default 0 = none)
     # - Second value is minimum height of the object in points, an int value in the range 1-65535. (Default 0 = none)
+    # - Third value is a boolean where TRUE specify that the first two values are the fixed image cell size.
+    #   (Default FALSE) Other string values, e.g. FIXED are also interpreted as TRUE.
     if not size or not isinstance(size, list):
-        return ''
-    return ((f' width="{size[0]}"' if len(size) > 0 and size[0] else '')
-        + (f' height="{size[1]}"' if len(size) > 1 and size[1] else ''))
+        return [''] * 3
+    return [f' width="{size[0]}"'  if len(size) > 0 and size[0] else '',
+            f' height="{size[1]}"' if len(size) > 1 and size[1] else '',
+             ' fixedsize="TRUE"'   if len(size) > 2 and size[2] else '']
 
 
 def expand(yaml_data):