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_canvas_stub.cpp

@@ -0,0 +1,136 @@
+/*
+ * 3D Canvas stubs for KiCad WASM build
+ * When KICAD_BUILD_3D_VIEWER_WASM=OFF, these provide stub implementations
+ * for 3D canvas and board adapter classes.
+ */
+
+#include <3d_canvas/eda_3d_canvas.h>
+#include <3d_canvas/board_adapter.h>
+#include <3d_viewer/eda_3d_viewer_settings.h>
+#include <gal/hidpi_gl_3D_canvas.h>
+#include <tool/tool_dispatcher.h>
+
+// EDA_3D_CANVAS stub implementations
+
+EDA_3D_CANVAS::EDA_3D_CANVAS( wxWindow* aParent, const wxGLAttributes& aGLAttribs,
+                               BOARD_ADAPTER& aSettings, CAMERA& aCamera,
+                               S3D_CACHE* a3DCachePointer ) :
+    HIDPI_GL_3D_CANVAS( aParent, aGLAttribs, aCamera, wxID_ANY )
+{
+    (void)aSettings;
+    (void)a3DCachePointer;
+    m_3d_render = nullptr;
+    m_parentStatusBar = nullptr;
+    m_parentInfoBar = nullptr;
+    m_eventDispatcher = nullptr;
+}
+
+EDA_3D_CANVAS::~EDA_3D_CANVAS()
+{
+}
+
+void EDA_3D_CANVAS::SetEventDispatcher( TOOL_DISPATCHER* aEventDispatcher )
+{
+    m_eventDispatcher = aEventDispatcher;
+}
+
+void EDA_3D_CANVAS::ReloadRequest( BOARD* aBoard, S3D_CACHE* aCachePointer )
+{
+    (void)aBoard;
+    (void)aCachePointer;
+    // No-op: 3D canvas not available in WASM
+}
+
+void EDA_3D_CANVAS::OnPaint( wxPaintEvent& aEvent )
+{
+    (void)aEvent;
+    // No-op: 3D canvas not available in WASM
+}
+
+void EDA_3D_CANVAS::DoRePaint()
+{
+    // No-op: 3D canvas not available in WASM
+}
+
+void EDA_3D_CANVAS::SetCurrentRender( RENDER_ENGINE aRenderEngine )
+{
+    (void)aRenderEngine;
+    // No-op: 3D canvas not available in WASM
+}
+
+void EDA_3D_CANVAS::RenderEngineChanged()
+{
+    // No-op: 3D canvas not available in WASM
+}
+
+void EDA_3D_CANVAS::Request_refresh( bool aRedrawImmediately )
+{
+    (void)aRedrawImmediately;
+    // No-op: 3D canvas not available in WASM
+}
+
+void EDA_3D_CANVAS::DisplayStatus()
+{
+    // No-op: 3D canvas not available in WASM
+}
+
+bool EDA_3D_CANVAS::doReRender()
+{
+    return false;
+}
+
+
+// BOARD_ADAPTER stub implementations
+
+BOARD_ADAPTER::BOARD_ADAPTER() :
+    m_board( nullptr ),
+    m_3dModelManager( nullptr ),
+    m_Cfg( nullptr )
+{
+}
+
+BOARD_ADAPTER::~BOARD_ADAPTER()
+{
+}
+
+
+// EDA_3D_VIEWER_SETTINGS stub implementations
+
+EDA_3D_VIEWER_SETTINGS::EDA_3D_VIEWER_SETTINGS() :
+    APP_SETTINGS_BASE( "3d_viewer", 3 )
+{
+}
+
+EDA_3D_VIEWER_SETTINGS::~EDA_3D_VIEWER_SETTINGS()
+{
+}
+
+
+// LAYER_PRESET_3D stub
+LAYER_PRESET_3D::LAYER_PRESET_3D( const wxString& aName ) :
+    name( aName )
+{
+}
+
+
+// PARAM_LAYER_PRESET_3D stub
+PARAM_LAYER_PRESET_3D::PARAM_LAYER_PRESET_3D( const std::string& aPath,
+                                               std::vector<LAYER_PRESET_3D>* aPresetList ) :
+    PARAM_LAMBDA<nlohmann::json>(
+        aPath,
+        [this]() { return presetsToJson(); },
+        [this]( const nlohmann::json& aJson ) { jsonToPresets( aJson ); },
+        nlohmann::json::array() ),
+    m_presets( aPresetList )
+{
+}
+
+nlohmann::json PARAM_LAYER_PRESET_3D::presetsToJson()
+{
+    return nlohmann::json::array();
+}
+
+void PARAM_LAYER_PRESET_3D::jsonToPresets( const nlohmann::json& aJson )
+{
+    (void)aJson;
+}