Fix SVG MIME type for embedded images (port of upstream PR #443)

SVG images embedded in connector/cable nodes failed to display because
the MIME type was image/svg instead of image/svg+xml per W3C spec.

src/wireviz/wv_output.py

@@ -15,7 +15,7 @@ from wireviz.wv_utils import (
     smart_file_resolve,
 )
 
-mime_subtype_replacements = {"jpg": "jpeg", "tif": "tiff"}
+mime_subtype_replacements = {"jpg": "jpeg", "svg": "svg+xml", "tif": "tiff"}
 
 
 # TODO: Share cache and code between data_URI_base64() and embed_svg_images()