The passed inX and inY x/y offset for readPixel -> [textureData] buffer, must be passed to readTexture.updateSubImage(..)'s destination offset. I.e. the buffer's content starting at beginning is filled via glReadPixels w/ offset applied. Hence the texture update must pass the offset as the destination offset, i.e. to be used for glTexSubImage2D's offset. Currently zero is passed.
commit 3d94fac862c3605d3a0862e8e6f7725483398163: fixed as described