Some external tools seem to have issues with the definition
of Collada <transparency> - a float value in range (0,1).
However it is possible to use the <transparent> color as a container
for the <transparency> value. This seems to be a more reliable
method to export transparency values from Blender PBSDF Shaders.
The relevant documentation is in the collada 1.14 reference manual,
page 7-5 about the usage of transparent and transparency.
This fix makes export and import of the <transparency>
and <transparent> values more convenient and more reliable.