map.c: remove unnecessary explicit -DHAVE_SGX
map.c: code commentary improvements
map.c: fix a bug in get_dimensions()

add missing NULL-check
map.c: improve get_dimensions()

  -- also try FBIOGET_VSCREENINFO if HAVE_FB
  -- hardwire dimensions if HAVE_FREMANTLE
map.c: fix indentation of 'G'
masstag: fix ffmpeg invocation
map.c: introduce the concept of default size

  -- add $DfltWidth and $DfltHeight
  -- initially they are the same as $DpyWidth and $DpyHeight
  -- but -n changes them to reflect the size of the new window
  -- or can be set explicitly with -G dflt={<size>|<window>}
  -- the meaning of "fs" is changed to refer to the default size
  -- "FS" in <geo> will refer to $Dpy*
map-dev.c: sync with map.c