Fix overlays stacking on top of each other or not moving to the edge of
the screen when enabling or disabling overlays while emulation is
active.
This change only applies when Config::GFX_MOVABLE_PERFORMANCE_METRICS is
False.
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.
