@Override
    public void setMainFrameBufferOverride(FrameBuffer fb) {
        mainFbOverride = null;
        if (context.boundFBO == 0) {
            // Main FB is now set to fb, make sure its bound
            setFrameBuffer(fb);
        }
        mainFbOverride = fb;
    }
Copy the source buffer to the destination buffer, including both color
and depth.

@param src the source buffer (unaffected)
@param dst the destination buffer
jme3-core/src/main/java/com/jme3/renderer/opengl/GLRenderer.java