Remove unused variable count in compute_average_rgb
Fix savegame regression from 03799d88ca9c22717afd8895a686d0f222e7efe1

Commit 03799d88ca9c22717afd8895a686d0f222e7efe1 converted from int to
segnum_t some fields that are copied to disk raw.  This changed the
format of savegames.  Worse, point_seg is saved via raw copy, but
reloaded piecewise, so savegames written using
03799d88ca9c22717afd8895a686d0f222e7efe1 or later cannot be reloaded
correctly.  This commit splits point_seg into point_seg and
point_seg_rw, and reverts the other incorrect changes.
Fix savegame regression from ab6598f7b6ca9b729be4bc62cdf5aacb4e5d2032

Commit ab6598f7b6ca9b729be4bc62cdf5aacb4e5d2032 changed MarkerObject
from int[] to objnum_t[], which has different size.  Copy the data back
into an int[] for saving to maintain compatibility.
Fix savegame regression from ab6598f7b6ca9b729be4bc62cdf5aacb4e5d2032

Commit ab6598f7b6ca9b729be4bc62cdf5aacb4e5d2032 changed
Dead_controlcen_object_num and Escort_goal_index from int to objnum_t,
which has different size.  Copy the data back into an int for saving to
maintain compatibility.