mx: NodeFactory: support creating blank rect from image

Image nodes are nice to have in some schematics.
Because of the representation in mxGraph, it would be a pain to create them in the xml.

Instead, this follows the least-effort methodology and replace the image by an equally sized rectangle.

In drawio GUI, the image can then be switched back in place of the rectangle.

As a reference on the mxImageShape if someone wants to dive deeper:
https://jgraph.github.io/mxgraph/java/docs/com/mxgraph/shape/mxImageShape.html

graphviz2drawio/mx/NodeFactory.py

@@ -29,6 +29,14 @@ def rect_from_svg_points(self, svg):
                 height = test_height
         return Rect(x=min_x, y=min_y, width=width, height=height)
 
+    @staticmethod
+    def rect_from_image(attrib):
+        filtered = dict()
+        for k, v in attrib.items():
+            if k in ['x', 'y', 'width', 'height']
+                filtered = { k: float(v.strip('px')) }
+        return Rect(**filtered)
+
     def rect_from_ellipse_svg(self, attrib):
         cx = float(attrib["cx"])
         cy = float(attrib["cy"])
@@ -56,6 +64,8 @@ def from_svg(self, g):
             rect = self.rect_from_svg_points(
                 SVG.get_first(g, "polygon").attrib["points"]
             )
+        elif SVG.has(g, "image"):
+            rect = self.rect_from_image(SVG.get_first(g, "image").attrib)
         else:
             rect = self.rect_from_ellipse_svg(SVG.get_first(g, "ellipse").attrib)