It looked a bit out of place wedged in between the FPS and VPS.

fb4ff3e added a statistics option to show the internal resolution, but
it just showed the total size of the EFB (which is always 640x528) times
the IR scale, so it didn't convey any useful information.

This commit instead makes the option use the size of the last XFB copy
(not multiplied by the IR scale), which changes based on the game's
rendering resolution.

This avoids the static initialization order fiasco between Core and VideoCommon

Co-authored-by: Jordan Woyak <jordan.woyak@gmail.com>

Use the normal state changed `HookableEvent` instead of having
`Core::NotifyStateChanged` call `g_perf_metrics.OnEmulationStateChanged`
directly.

The direct call was added in bad78cfed416d89ceb6b0acf6c134d3691b3d624 to
avoid a crash. At the time state changed callbacks were stored in a
vector, and the crash was caused by `g_perf_metric`'s destructor trying
to remove the callback from the already-destroyed vector.

Later a97627e736ff352e031123519c799e65cc98b32c switched state changed
callbacks to use `HookableEvent`, which is specifically designed to
handle the case where a hook outlives its associated event.

Since the workaround is no longer necessary replace it with a standard
`EventHook`.

Add a minimum value for the automatic size of the performance metrics
graph. The graph can still be manually resized smaller than this limit.

This prevents the graph from automatically resizing itself to be too
small to contain the full graph and legend, which happened when using
native resolution with `Auto-Adjust Window Size` enabled.

Fix a bug causing the performance graph to not resize when the render
window changed size:
* When changing the render window size during emulation the performance
  graph wouldn't update its size until the next emulation session.
* When changing the render window size with no emulation active (by
  changing the Internal Resolution with Auto-Adjust Window Size enabled)
  the performance graph wouldn't update its size until the second
  emulation session after the change.

Before explaining why the bug happened, here are some details about Dear
ImGui (henceforth ImGui) for context:
* In order to allow programs to specify initial ImGui window sizes while
  also allowing the user to resize them, `SetNextWindowSize` takes a
  flag from the `ImGuiCond_` enum specifying under what circumstances
  that function should actually have any effect.
* ImGuiCond_FirstUseEver causes ImGui to only apply the command when the
  window doesn't have any saved size information for that session or in
  the ini file specified by `ImGui::GetIO().IniFilename`. Since we set
  that filename to `nullptr`, in practice the resize command is applied
  on the first frame of each ImGui/emulation session.
* Qt saves the most recent size of the render window across emulation
  (and even Dolphin) sessions, which is then used to set the initial
  value of `ImGui::GetIO().DisplaySize` in the next emulation session.
* It takes multiple frames for the size of the render window to update
  when changed by setting the internal resolution. This means that
  `ImGui::GetIO().DisplaySize` will have a stale value in the
  intervening frames, and specifically for the first few frames of
  emulation if the resolution was changed beforehand.

When changing the resolution during emulation the call to
`SetNextWindowSize` had no effect because of the
`ImGuiCond_FirstUseEver` flag. `DisplaySize` would be updated several
frames later, and then the next emulation session would update the graph
size on its first frame.

When changing the resolution outside emulation and then starting a game,
the call to SetNextWindowSize on the first frame took effect but used
the stale value of `DisplaySize`. `DisplaySize` would be updated a few
frames later, but the graph wouldn't be resized until the first frame of
the second emulation session.

This commit fixes the issue by using the `ImGuiCond_Always` flag in the
performance graph's call to `SetNextWindowSize` when the render window
size changes.

Add vertical padding between the performance graph (when it's enabled)
and the FPS/VPS/Speed overlays.