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

Core/Util/PathUtil.cpp

@@ -141,6 +141,8 @@ Path GetSysDirectory(PSPDirectories directoryType) {
 		return PreferUpperCaseDir(pspDirectory, "SHADERS", "shaders");
 	case DIRECTORY_CUSTOM_THEMES:
 		return PreferUpperCaseDir(pspDirectory, "THEMES", "themes");
+	case DIRECTORY_CUSTOM_OVERLAYS:
+		return PreferUpperCaseDir(pspDirectory, "OVERLAYS", "overlays");
 	case DIRECTORY_NAND:
 		return pspDirectory / "NAND";