Fixed some texture loading errors

[JBKacerovsky]

Small changes to fix texture loading errors that caused crashes when converting some glb models loaded with trimesh to pyrender scenes using pyrender.Scene.from_trimesh_scene

• material.py: updated baseColorTexture.setter to check for "L" and "LA" mode textures and convert them to RGBA (avoids ValueError('Cannot reformat 2-channel texture into RGB') for greyscale baseColorTextures
• utils.py: updated format_texture_source to handle bool textures to aviod TypeError('Invalid type {} for texture'.format(
  type(texture)