how do i draw the text attachment of an fbo to the default frame buffer?

You have to set the texture minifying function (GL_TEXTURE_MIN_FILTER) by glTexParameter:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

If you are not generating mipmaps (with glGenerateMipmap) it is important to set GL_TEXTURE_MIN_FILTER. Since the default filter is GL_NEAREST_MIPMAP_LINEAR, the texture would be “Mipmap Incomplete” if you did not change the minimize function to GL_NEAREST or GL_LINEAR.

You missed to multiply gl_FragCoord.xy by rdx in the assignment shader:

FragColor = texture2D(mytexture, gl_FragCoord.xy);

FragColor = texture2D(mytexture, gl_FragCoord.xy * rdx);