properly handle the hotbar item dropping logic, server-side

i wasn't being clear on the path to follow between the server and world,
when told to drop an item. also properly handle bogus info and
do other api checks..

nothing happens clientside (and it will assert actually if you do)
initial dropping (somewhat) works, although not visible

decrements the stack sizes properly, except in some cases when
the size is 1 already..sync failure it seems, or some client
nonsense more likely.

also add some brief code on nearly getting rendering of dropped
items
fix off by one error (eg wouldn't let you drop an item of 1 count)
dropped items now works well, incl. rendering (as blue, for now)

will need to add a nice rotation animation and glow, probably.