Convert Projected Tetrahedra class to OpenGL2 backend

Switch to using the exp function directly in the fragment shader
as opposed to building and sending down a texture map. New valid
image is very slightly more transparent than previous, although
the existing valid images for this test show significant variances
between themselves.

Change-Id: I68fce12dbc08a6986595af501bb449f6dfdc1ac8
Merge topic 'projected_tyet' into master

c7fa3a65 Convert Projected Tetrahedra class to OpenGL2 backend