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/Map.h

@@ -95,7 +95,6 @@ public:
     static const int areamap[20 * 20];
     int contents[40 * 30];
     bool explored[20 * 20];
-    int vmult[30];
 
     bool isexplored(const int rx, const int ry);
     void setexplored(const int rx, const int ry, const bool status);