I want to render to this texture again, without clearing its data. If I clear it every frame and render to it then everything looks ok, but if I don't clear it (don't call framebuffer.clear() with this texture as render target) then I get dirty texture on input - it is filled with trash and I don't know why.
Of course i clear it when it is first time created, but i don't want to do it every rendered frame.
For example I have rendered letter "A" to this texture and in next animation frame I want to add "B", if I clear this texture then I have to render both letters, but I want to add just new letter - imagine when there are 1000 letters and you want to add 1, if you clear this texture then you have to render 1000 previous letters to add 1 new on the end. I want to avoid it.