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).
Constants.h will house constants like the screen size and others. But
basically only the screen size for now.
Now we don't have to type that "4 bytes per 40 chars (whole screen)"
comment everywhere...
