NudgeOutOfSolid: fix "random" failures on slightly inclined planes

NudgeOutOfSolid: fix "random" failures on slightly inclined planes

History:
This epsilon value was first added to the nudge distance in
31e7715cffcebfd400b8473f3b2d3dc80937fc5a
and was replaced with bbox expansion in
25c09ca585e81bf9eb236a339ae5dcd64f63b679

With the expansion approach we need to compare to separation to make
"good location" detection reliable, partly to avoid float precision
problems, partly to avoid compatibility problems with normal TraceBox.
For example: we drop an entity to floor successfully with TraceBox, but
then NudgeOutOfSolid decides it's in solid.

Also updated the startsolid conditions to suit the 2-pass approach.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>