Add overlay (bezel) support

Draws a user image around or behind the PSP screen, to fill the black bars
on displays whose aspect ratio doesn't match the PSP's (#20325).

- Overlays are PNGs in PSP/OVERLAYS. A plain PNG there just works: it's
  drawn in front of the game, and its transparent hole is found by walking
  from the image center to each edge until a nearly opaque pixel. Existing
  RetroArch overlay images can be dropped in as-is.
- A subfolder with an overlay.ini can define overlays with a Name, Layer
  (Foreground/Background) and an explicit Hole=x,y,w,h.
- "Load overlay image..." in the display layout screen imports any PNG
  (including Android content URIs) into the overlays folder and selects it.
- The overlay is fit to the screen (aspect preserved, centered). The hole is
  mapped with the same scale, and the game is fit into it with the normal
  stretch / integer scale / aspect ratio options. Offset and scale don't
  apply while an overlay with a hole is active. This goes through
  CalculateDisplayOutputRect, so touch mapping and post shaders agree.
- The selection is a per-orientation, per-game DisplayLayout setting.
- Disabled in VR, Cardboard, and with skip buffer effects (except software
  rendering), since the presentation path doesn't run there.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QGBg32EFPsTXjAFu7dnTcz

UI/ImDebugger/ImDebugger.cpp

@@ -911,6 +911,7 @@ static void DrawInternals(ImConfig &cfg) {
 		{DIRECTORY_EXDATA, "EXDATA"},
 		{DIRECTORY_CUSTOM_SHADERS, "CUSTOM_SHADERS"},
 		{DIRECTORY_CUSTOM_THEMES, "CUSTOM_THEMES"},
+		{DIRECTORY_CUSTOM_OVERLAYS, "CUSTOM_OVERLAYS"},
 	};
 
 	if (ImGui::CollapsingHeader("GetSysDirectory")) {