ISM for polyhedral room has wrong transmission coefficients

[fakufaku]

The image source model for general polyhedral rooms produces wrong transmission coefficients.

import numpy
import numpy as np
import pyroomacoustics as pra

if __name__ == "__main__":

    # shoebox room: working correctly
    room = pra.ShoeBox([5, 7, 4], max_order=1, materials=pra.Material(0.1))
    # general room: not working
    room2 = pra.Room(room.walls, max_order=1)

    source_loc = [2, 3.5, 1.8]
    mic_loc = [3.7, 5.5, 1.0]

    room.add_source(source_loc)
    room2.add_source(source_loc)

    room.add_microphone(mic_loc)
    room2.add_microphone(mic_loc)

    room.image_source_model()
    room2.image_source_model()

    print("Shoebox damping:", np.sort(room.sources[0].damping))
    print("General damping:", np.sort(room2.sources[0].damping))

The output of this script should be the same transmission coefficients for the shoebox ISM and general ISM. However, this is the current output.

Shoebox damping: [[0.94868326 0.94868326 0.94868326 0.94868326 0.94868326 0.94868326  1.        ]]
General damping: [[0.7289998  0.7684333  0.8099998  0.85381484 0.8999999  0.94868326  1.        ]]

[fakufaku]

Fixed in #237