Add support to collada loader for all 4 opaque modes

Note: this changes the default transparency behavior (be sure to call it out in release notes - some collada files may need to change)

This change should bring us in line with the [collada 1.5.0 spec](http://www.khronos.org/files/collada_1_5_release_notes.pdf) - see pg 16.

This does have an effect on two of the examples:

 - `loader / collada / keyframe` shows a little gray tesselated sphere at the center of the scene that used to be transparent
 - `loader / collada / skinning` shows some specular hotspots on the character that weren't there before.  That one baffles me.  @tparisi, do you have any idea why that might have happened?