Merges another image into this one at the specified edge. For left/right merging, the heights must match. For top/bottom, the widths must match.
Examples
cube <- generate_cuboid(c(0, 0, 0), c(1, 1, 1))
sphere <- generate_sphere(c(0, 0, 0), radius = 0.5)
left <- render_mesh(sphere$vertices, sphere$triangles)
right <- render_mesh(cube$vertices, cube$triangles)
merged <- image_merge(left, right, "right")