Add engine-debug-tools and fix critter animation rendering bugs

- engine-debug-tools/: a debug dump hotkey + dump2png to inspect
  decoded critter frames independent of the renderer (see root
  README.md's "Debugging" section).
- engine-patches/04,05: fix critter sprite scale and an OpenGL
  texture cache collision, found using that tooling.
- mdlp: fix encodeFrames RLE-diffing upscaled frames against a
  previous-frame reference the real RSD8 decoder never implements
  (verified against Night Dive's original Mac source down to the
  68K assembly) - was rendering as scattered static in-game.
- Upscale pipeline: better color quantization (dithering, pure-ramp
  exclusion) and per-resource model selection.

build-image/build-engine.sh

@@ -42,6 +42,17 @@ for p in "$REPO_ROOT"/engine-patches/*.patch; do
     patch -p1 -d "$SCRATCH_ENGINE" < "$p"
 done
 
+# ENGINE_DEBUG_TOOLS=1 (set by the Makefile's dist_debug target) additionally
+# applies engine-debug-tools/engine-patches/ - debug-only instrumentation
+# (see root README.md's "Debugging" section) that stays out of every normal
+# build.
+if [ -n "${ENGINE_DEBUG_TOOLS:-}" ]; then
+    echo "== ENGINE_DEBUG_TOOLS set - also applying engine-debug-tools/engine-patches/ =="
+    for p in "$REPO_ROOT"/engine-debug-tools/engine-patches/*.patch; do
+        patch -p1 -d "$SCRATCH_ENGINE" < "$p"
+    done
+fi
+
 cd "$SCRATCH_ENGINE"
 
 echo "== Wiring up prebuilt SDL2/SDL2_mixer/fluidsynth-lite from the image =="