Pure python PNG writer

Add @lcampagn 's `make_png`. No need to import from another library.
Just `from vispy.util import make_png`
Since this is now used it two places (`test_dataio` and `test_image`), you should refactor both sets of code to import the `requires_` lib from the same place (probably `vispy/testing/_testing.py` made accessible from the `vispy.testing` namespace). Copying code is generally a bad idea, it's better to refactor to have both sets of tests pull from the same place.