Shader packs: define MC_NORMAL_MAP and MC_SPECULAR_MAP

OptiFine defines both when its normal and specular map options are on (its
default), and Iris 1.11.7 defines both too (observed with a probe pack of our
own: its final pass paints one stripe per macro). Horizon defined neither, so
packs took their no-normal-map branches; Bliss declares FlatNormals only in
the other branch and its terrain, block and entity programs failed with
"Unknown variable: FlatNormals".

Fixes #6811

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-shaders/src/macros.rs

@@ -99,6 +99,10 @@ impl Macros {
         m.define("MC_SHADOW_QUALITY", "1.0");
         m.define("MC_HAND_DEPTH", "0.125");
         m.define("MAX_COLOR_BUFFERS", "16");
+        // The normal and specular maps are on (OptiFine's shaders.txt, H: their options, on by
+        // default; Iris 1.11.7 defines both too, with or without a resource pack's maps).
+        m.define("MC_NORMAL_MAP", "");
+        m.define("MC_SPECULAR_MAP", "");
         // Packs take Iris's extensions (compute programs, images, custom textures) where this
         // is defined.
         m.define("IS_IRIS", "");
@@ -329,6 +333,32 @@ pub(crate) fn is_ident(c: char) -> bool {
 mod tests {
     use super::*;
 
+    /// Packs take their normal- and specular-map branches, as they do with the maps on: a
+    /// declaration made only there is seen.
+    #[test]
+    fn programs_take_the_normal_and_specular_map_branches() {
+        let fsh = "#version 330\nout vec4 c;\n\
+                   #ifdef MC_NORMAL_MAP\nconst float bump = 1.0;\n#endif\n\
+                   #ifdef MC_SPECULAR_MAP\nconst float shine = 2.0;\n#endif\n\
+                   void main() { c = vec4(bump, shine, 0.0, 1.0); }\n";
+        let vsh = "#version 330\nvoid main() { gl_Position = vec4(0.0); }\n";
+        let pack = crate::Pack::from_zip_bytes(
+            "P.zip",
+            crate::pack::tests::zip(&[
+                ("shaders/composite.vsh", vsh),
+                ("shaders/composite.fsh", fsh),
+            ]),
+        )
+        .unwrap();
+        let m = Macros::standard();
+        let program = crate::translate_composite(
+            &crate::preprocess(&pack, "composite.vsh", &m),
+            &crate::preprocess(&pack, "composite.fsh", &m),
+            crate::Target::Buffers,
+        );
+        assert!(program.is_ok(), "{:?}", program.err());
+    }
+
     #[test]
     fn iris_features_are_defined_or_noted() {
         let properties = crate::Properties::parse(