Textures shouldn't be cleared with a full opacity color.

Is there any special reason why inside ````ofxFXObject::setTexture```` and ````ofxFXObject::begin````, ````ofClear```` is called with a full 255 alpha?

For my usage this has proven a big problem, as it is impossible to pass in a transparent texture, and keep the result transparent.

This pull request simply sets the alpha to zero when clearing the textures.