[designspace] Changing the default value of an axis causes trouble

[justvanrossum]

Steps:

• create new (.designspace) font
• add glyph
• add axis
• change default value of axis
• add source at axis for glyph

Traceback (most recent call last):
  File "/Users/just/code/git/BlackFoundry/fontra/src/fontra/core/fonthandler.py", line 135, in _processWritesOneCycle
    await writeFunc()
  File "/Users/just/code/git/BlackFoundry/fontra/src/fontra/backends/designspace.py", line 564, in putGlyph
    self._createDefaultSourceAndUFO(sourceName)
  File "/Users/just/code/git/BlackFoundry/fontra/src/fontra/backends/designspace.py", line 693, in _createDefaultSourceAndUFO
    assert not self.dsSources
           ^^^^^^^^^^^^^^^^^^
AssertionError