droptofloor(): refactor and use correct conditions for each BSP format

droptofloor(): refactor and use correct conditions for each BSP format

Behaviour changes:

Although startsolid is used by Q3 and Q2, it's too strict for Quake
which used allsolid, so we now use the correct conditions for each BSP
format.

The downtrace endpoint is now calculated after
sv_gameplayfix_droptofloorstartsolid_nudgetocorrect.

The downtrace endpoint is now offset when using the
sv_gameplayfix_droptofloorstartsolid fallback path, otherwise it would
trace a diagonal line for entities whose origin is not horizontally centred
in their bbox.

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