CFrame: Only re-assign focus when "Render to main" is enabled.

It may cause fullscreen loops and since m_RenderFrame == m_RenderParent now it's not necessary to re-assign focus anymore.

No longer conflicts with PR #683, that PR has been rewritten.
Just wondering but is it possible at some point in the future to merge the two of these codepaths together to use the wx one, or is that not possible with the way Windows manages it's windows?