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Fix for ICET compositing with non black backgrounds OpenGL2

Ken Martin requested to merge ken-martin/paraview:icet_opengl2_fix into master

Fixes transparent compositing when using backgrounds other than black. Was a royal PITA to track down, seriously a PITA.

Basically background color was getting set on the FB and then translucent geometry rendered on top of it. FB captured and then composited with other FBs using a blend function assuming alpha weighted colors (which is usual). But the background color is not alpha weighted so two FBs, each with some background color with translucent geometry on top can composite into something that sums up to more than RGB 1,1,1. Producing wacky errors. Basically the net effect was that the background color per pixel was getting multiplied by the number of FBs that had translucent geometry on that pixel. If any FB had opaque geometry on top then it would hide the error. If there was no translucent geometry the issue does not happen. If the background color is black the issue does not happen.

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