Fix several bugs in the Eraser tool

This MR solves several issues that affected the Eraser tool:

• One of the most embarassing Inkscape issues: #540 (closed) Eraser tool draws instead of erasing is now fixed. As requested by users commenting on the issue, attempting to erase a bitmap in "Cut" mode shows a warning in the status bar, but does not erase anything.

• Inkscape no longer crashes when the Eraser is used in the "Clip" mode on an object with a clip-path applied if that object additionally has clones which are selected alongside it – issue #1457 (closed).

• Inkscape no longer crashes when the Eraser is used in the "Delete" mode on a selection containing several shapes – issue #2283 (closed).

• Debug builds no longer crash when the user draws an eraser stroke so short that it doesn't contain any Bézier segments – issue #3308 (closed).

• Inkscape no longer crashes when the Eraser tool is used in the "Cut" mode on a rectangle with live path effects applied – issue #3310 (closed).

• The "Cut" mode of the Eraser can now erase from paths that contain only two nodes, such as bigons (leaf shapes) and ellipse segments. It also works correctly on ellipses which consist of only two arcs, such as the circles created with the "Geometry tool".

Fixes #540 (closed) | Fixes #1457 (closed) | Fixes #2283 (closed) | Fixes #3272 (closed) | Fixes #3308 (closed) | Fixes #3310 (closed)