Avoid some bad combinations of default values

- Avoid setting image.fixedsize default True when image.scale is
  specified different than "true" or "both" by the user.
- Avoid calculating the missing dimension unless image.fixedsize
  is actually True. It might have been specified False by the user.

src/wireviz/DataClasses.py

@@ -28,8 +28,11 @@ class Image:
                 else     "true" # When only one dimension is specified.
 
         if self.fixedsize is None:
-            self.fixedsize = self.width or self.height
+            # Normally equal to (self.width or self.height), but the user might override self.scale.
+            self.fixedsize = str(self.scale).lower() in ["true", "both"]
 
+        if self.fixedsize:
+            # If only one dimension is specified, compute the other because both are required.
             if self.height:
                 if not self.width:
                     self.width = self.height # Assuming 1:1 aspect ratio for now.