Do not corrupt textures between FBOs

Previously, if we set a texture (e.g. tex0) and some parts of it
were transparent, all the transparent things would have been changed
when doing fxobject[0].draw(...). The transparent parts would have
had their alpha channel mixed with the black color due to the
ofClear(0,0) call in setTexture. Now we no longer destroy textures.
Merge pull request #18 from OliverUv/fix-transparency

Do not corrupt textures between FBOs