Collada import: use getOpacity() instead of getTransparent()

OpenCollada (for whatever reason) will not report a <transparent>
texture through getTransparent(), it will only report it through
getOpacity(), so this is neccessary to make transparent textures
work. Before this, the code to make a texture node never ran.

For constants getOpacity() has already multiplied the <transparent>
and <transparency> for us.

Here is a file that tests combinations of <transparent>/<transparency> constants.

You can check it's the same as master.