#87: mech armour PANELS now darken with damage -- the .DZM material system was loaded and unused

Players: "the actual enemy mech in external view is not showing darkened armor
panels".  We swapped destroyed LIMB meshes but never darkened a panel.

The 1995 game darkens armour through the MATERIALS.  MakeMechRenderables
(FUN_004cef28) built, per (damage zone, material), a watcher
FUN_004573e4(material, &zone->damageLevel, 0.1f) that snapshotted the materials

engine/MUNGA_L4/bgfload.cpp

@@ -619,6 +619,9 @@ struct Builder {
     uint32_t currentColor = 0xFFB0B0B8u;
     bool currentHasDiffuse = false;    // material carried an explicit diffuse tag
     std::string currentTex;
+    // authored (pre-substitution) "library:material_mtl" of the material in force --
+    // the .DZM armour-damage lists key on this (issue #87)
+    std::string currentMatName;
     int currentTexChannel = 0;         // BSL bit-slice (BMF tag 0x18)
     bool currentTexScroll = false;     // TEXTURE SPECIAL " SCROLL" (tag 0x2037)
     float currentTexScrollU = 0.0f, currentTexScrollV = 0.0f;
@@ -872,6 +875,7 @@ struct Builder {
             batch.color = pureEmissive ? currentColor          // keep BLACK: L4D3D's
                         : (useRamp ? rampTint : currentColor); // pure-emissive test keys on it
             batch.texPath = currentTex;
+            batch.matName = currentMatName;		// .DZM armour-damage key (issue #87)
             batch.texChannel = currentTexChannel;
             batch.texScroll  = currentTexScroll;
             batch.texScrollU = currentTexScrollU;
@@ -1185,6 +1189,9 @@ struct Builder {
                         size_t maxn = ch.len - 1, n = 0;
                         while (n < maxn && s[n]) ++n;
                         std::string full(s, n);
+                        // remember the AUTHORED name (before the paint callback rewrites
+                        // it) -- the .DZM zone material lists are authored against it
+                        currentMatName = full;
                         // baked ground-shadow material (basev:shadow_mtl etc.)
                         currentShadowMat = false;
                         for (size_t si = 0; si + 6 <= full.size(); ++si)