Add HTML template placeholder for diagram_png_base64

This will enable users to replace the SVG diagram with an embedded PNG,
that is an improved work-around when the SVG output from Graphviz
is not looking good. Suggested as work-around for Graphviz bug in
#175 (comment)

src/wireviz/svgembed.py

@@ -8,6 +8,20 @@
 mime_subtype_replacements = {"jpg": "jpeg", "tif": "tiff"}
 
 
+# TODO: Share cache and code between data_URI_base64() and embed_svg_images()
+def data_URI_base64(file: Union[str, Path], media: str = "image") -> str:
+    """Return Base64-encoded data URI of input file."""
+    file = Path(file)
+    b64 = base64.b64encode(file.read_bytes()).decode("utf-8")
+    uri = f"data:{media}/{get_mime_subtype(file)};base64, {b64}"
+    # print(f"data_URI_base64('{file}', '{media}') -> {len(uri)}-character URI")
+    if len(uri) > 65535:
+        print(
+            "data_URI_base64(): Warning: Browsers might have different URI length limitations"
+        )
+    return uri
+
+
 def embed_svg_images(svg_in: str, base_path: Union[str, Path] = Path.cwd()) -> str:
     images_b64 = {}  # cache of base64-encoded images
 

src/wireviz/wv_html.py

@@ -6,6 +6,7 @@
 
 from wireviz import APP_NAME, APP_URL, __version__, wv_colors
 from wireviz.DataClasses import Metadata, Options
+from wireviz.svgembed import data_URI_base64
 from wireviz.wv_gv_html import html_line_breaks
 from wireviz.wv_helper import (
     flatten2d,
@@ -81,7 +82,7 @@ def generate_html_output(
         "<!-- %fontname% -->": options.fontname,
         "<!-- %bgcolor% -->": wv_colors.translate_color(options.bgcolor, "hex"),
         "<!-- %diagram% -->": svgdata,
-        # TODO: "<!-- %diagram_png_base64% -->": base64 of png file
+        "<!-- %diagram_png_base64% -->": data_URI_base64(f"{filename}.png"),
         "<!-- %filename% -->": str(filename),
         "<!-- %filename_stem% -->": Path(filename).stem,
         "<!-- %bom% -->": bom_html,