Remove vmult lookup tables

There's really no need to put the y-multiplication in a lookup table.
The compiler will optimize the multiplication better than putting it in
a lookup table will.

To improve readability and to hardcode things less, the new
SCREEN_WIDTH_TILES and SCREEN_HEIGHT_TILES constant names are used, as
well as adding a new TILE_IDX macro to calculate the index of a tile in
a concatenated-rows (row-major in formal parlance) array. Also, tile
numbers are stored in a temporary variable to improve readability as
well (no more copy-pasting `contents[i + vmult[j]]` over and over
again).

desktop_version/src/Tower.h

@@ -23,7 +23,6 @@ public:
     short back[40 * 120];
     short contents[40 * 700];
     short minitower[40 * 100];
-    int vmult[40 * 700];
 
     bool minitowermode;
 };