Wall textures are now sampled from texture images

using the raycast positions, I can determine the x coordinate to sample
from on the texture just by modding the larger coordinate by one (making
the texture repeat every unit.) The Y coordinate is based on the
current calculated wall height (determined by the distance between the
raycast and the player.)

Next I will make textures loaded on an atlas instead of a single image
so I can make multiple different types of wall.