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Fix bug #543; POLYSTART and POLYEND no longer show modal dialog boxes for errors
Instead, they throw "Invalid State" errors.

Add a comment to explain that the argument to atan2 are reversed.

Relabel the inputs of ELLIPSEARC2 to match the documentation.
This brought to light the apparent mislabelling of x as y and vice-versa that had confused me before. I'm confident in the naming of the inputs, but not as much in the naming of the local variables.

The good news is that, even if the variable names are incorrect, the procedure still passes all tests.

Replace usage of ELIPSA2 with ARCOELIPSE2
ELIPSA2 is deprecated and ARCOELIPSE2 is the preferred name.

Add a regression test for a degenerate case of ELLIPSEARC2
x-semiaxis = 0 and the angle swept is only 10.

Remove trailing whitespace

Improve the dashed-line ellipse example for ELLIPSEARC2
- Don't draw the ellipse twice
- End with the pen state down

esta -> ésta to avoid ambiguity

Rename :principio.ángulo to :ángulo.de.partida
The new name feels like a more natural translation for "start.angle"

Update the documentation for ELLIPSEARC2 to match fix for Bug #494.