Speedup SDL version with footsteps:

- Moved the common code of the two version of `game::draw_footsteps` (sdl/curses) into a single function `game::calculate_footstep_markers` in game.cpp

- Moved the remaining code that adds the footsteps to `w_terrain` into game.cpp as it is the same for SDL (when not using `cata_tiles`) and curses: `game::draw_footsteps`.

- Skip drawing the footsteps with `game::draw_footsteps` when `cata_tiles` is used. Instead let `cata_tiles` draw the footstep itself.

The last step removes a second call of `wrefresh(w_terrain)` when `cata_tiles` is used (first in `game::draw_ter`, second in `game::draw_footsteps`).
`cata_tiles` draws the terrain when `wrefresh(w_terrain)` is called, this happens now once only. That means the previous code draw the whole terrain window twice each turn.