Always call StubHandler open() when opening StubImageFile

[radarhere]

In all of our subclasses of StubImageFile, _open ends with the following code, calling StubHandler.open()

Pillow/src/PIL/WmfImagePlugin.py

Lines 151 to 153 in 29ff5fc

	loader = self._load()
	if loader:
	loader.open(self)

This PR suggests moving that code out of the subclasses, and instead forcing it to be called when every StubImageFile is opened.