Refuse formats the add-on does not implement

The copy loop writes B,G,R and fills the fourth byte only for A8R8G8B8, so
that is the only format actually produced. Every other value fell through to
that same loop and returned true with output that does not match the request.

ADDON_IMG_FMT_A8 was the worst of them: one byte per pixel is asked for and
three are written. The previous commit's bounds check now refuses that, but
it should not be reached at all.

Latent today only because CTexture::LoadIImage() asks for A8R8G8B8.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: cinema-ONE

src/MPOPicture.cpp

@@ -160,6 +160,15 @@ bool MPOPicture::Decode(uint8_t* pixels,
                         unsigned int pitch,
                         ADDON_IMG_FMT format)
 {
+  // The copy loop below writes B,G,R and only fills the fourth byte for
+  // A8R8G8B8, so that is the only format implemented here.
+  if (format != ADDON_IMG_FMT_A8R8G8B8)
+  {
+    kodi::Log(ADDON_LOG_ERROR, "%s: Unsupported target format (%d)", __func__,
+              static_cast<int>(format));
+    return false;
+  }
+
   size_t image = 0;
   while (image < m_images)
   {