Clamp overlays to the render window (with some padding), reset their
positions when the render window changes sizes, and add a setting to
enable moving the overlays (off by default, .ini only for now).
Move ImGui::End() calls out of if(ImGui::Begin()) blocks.
Quoting from ImGui::Begin's function comment in imgui.cpp:
"You always need to call ImGui::End() even if false is returned."
In practice this didn't cause problems because the windows don't have
title bars and thus can't be collapsed, and so the block containing
::End would always run, but let's do it the right way.
