Use P_AproxDistance in dashmode speed calculations

Assuming we don't want to revert !1309 (merged) instead, this is another potential resolution for #466 (closed). It simply uses the previous calculation for player->speed in dashmode's code block.

Since neither player->speed nor player->rmomx/player->rmomy are updated between P_3dMovement and dashmode's block, it should control consistently with previous 2.2 patches.