Allow loading of WMF images at a given DPI

[radarhere]

Resolves #4223

WmfImagePlugin can load both WMF and EMF files. When loading WMFs, it sets the DPI to 72. This is an arbitrary number, because WMFs are like SVGs - they don't have an intrinsic DPI, and can be scaled to any resolution. The Python code just tells the C code what size is required.

This PR suggests that after opening an WMF file, but not an EMF, the following code be used to set the DPI to 144, for example, by adding a dpi argument to WmfImagePlugin load -

with Image.open("Tests/images/drawing.wmf") as im:
    im.load(144)

Feel free to suggest an alternative API.

[hugovk]

Let's also update the docs and add to release notes.

[radarhere]

I've updated the documentation and added it to the release notes - I've moved WMF from identify-only into read-only. The Windows read functionality was actually added in PIL 1.1.17, but I've removed the references to PIL versions.

[hugovk]

Perhaps add .. code-block:: python to the code blocks, merge when you're ready.

[radarhere]

Okay, done.