Enable OpenGL GAL and 3D viewer stubs for WASM

Changes:
- Add glu_wasm_impl.cpp: GLU tesselator using KiCad's earcut triangulation
- Add GL/glu.h: GLU header for WASM builds
- Add 3D viewer stubs for KICAD_BUILD_3D_VIEWER_WASM=ON
- Remove obsolete gl_canvas_stub.cpp (OpenGL now uses real impl)
- Remove obsolete glu_stub.c (replaced by glu_wasm_impl.cpp)
- Update build-pcbnew.sh comment about GLU implementation

The OpenGL GAL now works in WASM using WebGL with LEGACY_GL_EMULATION.
3D viewer is stubbed out but builds successfully.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wasm/stubs/3d_scenegraph_stub.cpp

@@ -0,0 +1,127 @@
+/*
+ * 3D Scenegraph stubs for KiCad WASM build
+ * When KICAD_BUILD_3D_VIEWER_WASM=OFF, these provide stub implementations
+ * for 3D cache and scenegraph classes.
+ */
+
+#include <3d_cache/3d_cache.h>
+#include <3d_cache/3d_plugin_manager.h>
+#include <wx/log.h>
+
+// S3D_CACHE stub implementations
+
+S3D_CACHE::S3D_CACHE() :
+    m_FNResolver( nullptr ),
+    m_Plugins( nullptr ),
+    m_project( nullptr )
+{
+}
+
+S3D_CACHE::~S3D_CACHE()
+{
+}
+
+bool S3D_CACHE::Set3DConfigDir( const wxString& aConfigDir )
+{
+    m_ConfigDir = aConfigDir;
+    return true;
+}
+
+bool S3D_CACHE::SetProject( PROJECT* aProject )
+{
+    m_project = aProject;
+    return true;
+}
+
+void S3D_CACHE::SetProgramBase( PGM_BASE* aBase )
+{
+    (void)aBase;
+    // No-op: 3D cache not available in WASM
+}
+
+SCENEGRAPH* S3D_CACHE::Load( const wxString& aModelFile, const wxString& aBasePath,
+                              std::vector<const EMBEDDED_FILES*> aEmbeddedFilesStack )
+{
+    (void)aModelFile;
+    (void)aBasePath;
+    (void)aEmbeddedFilesStack;
+    // 3D models not supported in WASM
+    return nullptr;
+}
+
+FILENAME_RESOLVER* S3D_CACHE::GetResolver() noexcept
+{
+    return m_FNResolver;
+}
+
+std::list<wxString> const* S3D_CACHE::GetFileFilters() const
+{
+    static std::list<wxString> emptyList;
+    return &emptyList;
+}
+
+void S3D_CACHE::FlushCache( bool closePlugins )
+{
+    (void)closePlugins;
+    // No-op: no cache in WASM
+}
+
+void S3D_CACHE::ClosePlugins()
+{
+    // No-op: no plugins in WASM
+}
+
+S3DMODEL* S3D_CACHE::GetModel( const wxString& aModelFileName, const wxString& aBasePath,
+                                std::vector<const EMBEDDED_FILES*> aEmbeddedFilesStack )
+{
+    (void)aModelFileName;
+    (void)aBasePath;
+    (void)aEmbeddedFilesStack;
+    // 3D models not supported in WASM
+    return nullptr;
+}
+
+void S3D_CACHE::CleanCacheDir( int aNumDaysOld )
+{
+    (void)aNumDaysOld;
+    // No-op: no cache in WASM
+}
+
+SCENEGRAPH* S3D_CACHE::checkCache( const wxString& aFileName, S3D_CACHE_ENTRY** aCachePtr )
+{
+    (void)aFileName;
+    if( aCachePtr )
+        *aCachePtr = nullptr;
+    return nullptr;
+}
+
+bool S3D_CACHE::getHash( const wxString& aFileName, HASH_128& aHash )
+{
+    (void)aFileName;
+    (void)aHash;
+    return false;
+}
+
+bool S3D_CACHE::loadCacheData( S3D_CACHE_ENTRY* aCacheItem )
+{
+    (void)aCacheItem;
+    return false;
+}
+
+bool S3D_CACHE::saveCacheData( S3D_CACHE_ENTRY* aCacheItem )
+{
+    (void)aCacheItem;
+    return false;
+}
+
+SCENEGRAPH* S3D_CACHE::load( const wxString& aModelFile, const wxString& aBasePath,
+                              S3D_CACHE_ENTRY** aCachePtr,
+                              std::vector<const EMBEDDED_FILES*> aEmbeddedFilesStack )
+{
+    (void)aModelFile;
+    (void)aBasePath;
+    (void)aEmbeddedFilesStack;
+    if( aCachePtr )
+        *aCachePtr = nullptr;
+    return nullptr;
+}