Always delete our BMs.

Also ensure finish_frontend is always called when Win32 main window is
closed.
The OAM is getting closer!
Add window rendering
Fix VS -.-
Really fix VS
Reduce duplication in MBC code
Merge branch 'mbc-rewrite'
Remark that windows use bg_char_sel
Fix positioning of OBJ sprites

It now segfaults though and I don't know why, because obj->x and obj->y
can take negative values.