Frametests: add framedump rendering test runner and PNG screenshot support

- Add frametests.py: walks a dump tree, renders each dump per config variant
  through PPSSPPHeadless, generates reference images when missing and compares
  MSE when present, and writes a self-contained HTML report. The JSON config
  (which lives with the test set, not in the repo) points at the data tree
  and defines variants as suffix -> CLI args, e.g. 'soft': '--graphics=software'.
- Headless: --screenshot-save saves PNG when the path ends in .png; new
  --screenshot-diff always writes a visual comparison when comparing;
  screenshot comparison failures (mismatch or unloadable reference) now fail
  the test instead of passing silently.
- Read back framebuffers top-down, flipping only for BMP output/input
  (fixes upside-down PNG references). Sync libretro copy accordingly.
- Document the system in docs/frametest.md; add AGENTS.md reference.

.gitignore

@@ -60,6 +60,9 @@ ppsspp.ini
 imgui.ini
 PPSSPPControls.dat
 
+# Frametest output (see frametests.py)
+frametest-out/
+
 # Gradle/Android Studio
 .gradle
 .idea
@@ -147,5 +150,5 @@ RAPrefs_PPSSPP.cfg
 
 # For CLion
 cmake-build-*/
-
-/.vscode/
+
+/.vscode/