Bugfixes: 1. Don't ask for pbuffer render-to-texture if the extension is not available.  2. Don't demand render-to-texture for all pbuffers, fall back on non-texture-capable.
Bugfixes: 1. Don't ask for pbuffer render-to-texture if the extension is not available.  2. Don't demand render-to-texture for all pbuffers, fall back on non-texture-capable.

Alexpux/Coin3D,Alexpux/Coin3D,Alexpux/Coin3D,Alexpux/Coin3D