Snap indicators flicker when path and bbox targets are enabled

Summary:

With certain snapping options and shapes, the snap indicator flickers as the mouse is moved.

This is a residual issue broken out of #4242 (closed). See the discussion at !5480 (comment 1462460008) for some more info.

Steps to reproduce:

• Open Inkscape.
• Create a rectangle.
• Ensure the rectangle has no stroke, is not rotated, and is selected.
• Ensure snapping is enabled, and both "Snap bounding box edges" and "Snap paths" are enabled.
• Using the node tool, move the mouse along the edges of the rectangle.

What happened?

The snap indicator sometimes disappears, giving the appearance of flickering.

What should have happened?

Snap indicator should always be there, and not flicker.

Version info

1.2.2 and 1.4 dev

Notes

The path and bounding box targets are the same shape, and are fighting with each other. The closest one to the mouse fluctuates randomly due to floating-point errors, and the indicator only appears if the path target is closer.

/cc @dvlierop