Feature request: re-enable stretching objects to zero width or height

[TimOphelders]

In older versions of Ipe, the "stretch objects" tool allowed stretching objects to a width or height of zero. I assume that this was disabled because of issue #437 or #477. I frequently used this functionality to project objects onto a horizontal or vertical line.

It seems that the current version of Ipe can still open and work with files that have objects (without elliptical segments) with a singular transformation matrix. I request that this feature is re-enabled, perhaps with an explicit check and error message if objects to be transformed contain elliptical segments.