Just don't clear your render target. In case, where you are drawing to back buffer, remove those functions:
glClearColor(...); glClear(GL_COLOR_BUFFER_BIT);
(if you rendering to any other target, you already know how to do it ;) )
Consider also leaving depth /stencil buffers uncleared:
`GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT`
But wait, is performance really so bad, that you gonna let a hairy mess been drawn up?