Falling tiles disappear in walls

[TCH68k]

Steps to reproduce:

1. Put some falling tiles above walls.
2. Make them fall.
3. They will not hit the ground beneath.

Illustration video: http://oscomp.hu/depot/sdlpop_falling_tiles_disappears_in_walls.mkv
(It also illustrates the bug described in #355.)

Note: The same happens in the original game, but i still think, this should be fixed.