[viewer] Enable changing color for list visual mesh

[iory]

Since visual_mesh is a list, it does not change even if you change the color of mesh, so I changed it to change.

from skrobot.viewers import TrimeshSceneViewer
from skrobot.models import PR2
import numpy as np



v = TrimeshSceneViewer()
r = PR2()
v.add(r)
v.show()

color = [255, 0, 0, 200]
for link in r.find_link_path(r.rarm_root_link, r.r_gripper_l_finger_tip_link) + r.find_link_path(r.rarm_root_link, r.r_gripper_r_finger_tip_link):
    if hasattr(link, '_visual_mesh') and isinstance(link._visual_mesh, list):
        if len(link._visual_mesh) == 0:
            continue
        m = link._visual_mesh[0]
    else:
        continue
    n_facet = len(m.visual.face_colors)
    m.visual.face_colors =  np.array([color] * n_facet)

[iory]

Please use set_color method to change link color
#338