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/GL/glu.h

@@ -0,0 +1,88 @@
+/**
+ * GLU header for WebAssembly builds
+ *
+ * Provides GLU function declarations. Implementation is in glu_wasm_impl.cpp
+ * which uses KiCad's earcut-based polygon triangulation.
+ */
+
+#ifndef __GLU_H__
+#define __GLU_H__
+
+#include <GLES2/gl2.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// GLU types
+typedef double GLdouble;
+typedef void (*_GLUfuncptr)(void);
+typedef struct GLUtesselator GLUtesselator;
+typedef struct GLUquadric GLUquadric;
+
+// GLU constants
+#define GLU_TESS_BEGIN          100100
+#define GLU_TESS_VERTEX         100101
+#define GLU_TESS_END            100102
+#define GLU_TESS_ERROR          100103
+#define GLU_TESS_EDGE_FLAG      100104
+#define GLU_TESS_COMBINE        100105
+#define GLU_TESS_BEGIN_DATA     100106
+#define GLU_TESS_VERTEX_DATA    100107
+#define GLU_TESS_END_DATA       100108
+#define GLU_TESS_ERROR_DATA     100109
+#define GLU_TESS_EDGE_FLAG_DATA 100110
+#define GLU_TESS_COMBINE_DATA   100111
+
+#define GLU_TESS_WINDING_RULE       100140
+#define GLU_TESS_WINDING_ODD        100130
+#define GLU_TESS_WINDING_NONZERO    100131
+#define GLU_TESS_WINDING_POSITIVE   100132
+#define GLU_TESS_WINDING_NEGATIVE   100133
+#define GLU_TESS_WINDING_ABS_GEQ_TWO 100134
+
+// Tesselator functions
+GLUtesselator* gluNewTess(void);
+void gluDeleteTess(GLUtesselator* tess);
+void gluTessCallback(GLUtesselator* tess, GLenum which, _GLUfuncptr fn);
+void gluTessProperty(GLUtesselator* tess, GLenum which, GLdouble value);
+void gluGetTessProperty(GLUtesselator* tess, GLenum which, GLdouble* value);
+void gluTessNormal(GLUtesselator* tess, GLdouble x, GLdouble y, GLdouble z);
+void gluTessBeginPolygon(GLUtesselator* tess, void* userData);
+void gluTessBeginContour(GLUtesselator* tess);
+void gluTessVertex(GLUtesselator* tess, GLdouble coords[3], void* data);
+void gluTessEndContour(GLUtesselator* tess);
+void gluTessEndPolygon(GLUtesselator* tess);
+const unsigned char* gluErrorString(GLenum error);
+
+// Quadric constants
+#define GLU_OUTSIDE             100020
+#define GLU_INSIDE              100021
+#define GLU_POINT               100010
+#define GLU_LINE                100011
+#define GLU_FILL                100012
+#define GLU_SILHOUETTE          100013
+#define GLU_SMOOTH              100000
+#define GLU_FLAT                100001
+#define GLU_NONE                100002
+
+// Quadric type aliases
+typedef GLUquadric GLUquadricObj;
+
+// Quadric functions (stubs - not implemented for WASM)
+GLUquadric* gluNewQuadric(void);
+void gluDeleteQuadric(GLUquadric* q);
+void gluQuadricDrawStyle(GLUquadric* q, GLenum style);
+void gluQuadricNormals(GLUquadric* q, GLenum normals);
+void gluCylinder(GLUquadric* q, double base, double top, double height, int slices, int stacks);
+void gluDisk(GLUquadric* q, double inner, double outer, int slices, int loops);
+void gluSphere(GLUquadric* q, double radius, int slices, int stacks);
+
+// Matrix functions (stub)
+void gluPerspective(double fovy, double aspect, double zNear, double zFar);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __GLU_H__ */