feat(webgl): Add WEBGL_GAL implementation (compiles, not yet rendering)

Copy and adapt OpenGL GAL to WebGL:
- Copy all OpenGL GAL source files to tests/gal-regression/wasm/webgl/
- Rename classes from OPENGL_* to WEBGL_*
- Add kiglew.h with WebGL2/GLES3 headers and GLEW stubs
- Add webgl_antialiasing.h/cpp adapted for WEBGL_COMPOSITOR
- Add shader generator (generate_shaders.py) for WASM build
- Update Makefile with all KiCad dependencies (C++20, GLM, clipper2, etc.)

Build produces 544KB JS + 4.9MB WASM. Tests run but render blank
(expected - GL context initialization needs WebGL adaptation).

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

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

tests/gal-regression/wasm/webgl/gl_context_mgr.cpp

@@ -0,0 +1,118 @@
+/*
+ * This program source code file is part of KiCad, a free EDA CAD application.
+ *
+ * Copyright (C) 2016 CERN
+ * Copyright The KiCad Developers, see AUTHORS.txt for contributors.
+ * @author Maciej Suminski <maciej.suminski@cern.ch>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, you may find one here:
+ * http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+ * or you may search the http://www.gnu.org website for the version 2 license,
+ * or you may write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
+ */
+
+#include "gl_context_mgr.h"
+#include <wx/debug.h>
+
+
+wxGLContext* GL_CONTEXT_MANAGER::CreateCtx( wxGLCanvas* aCanvas, const wxGLContext* aOther )
+{
+    wxGLContext* context = new wxGLContext( aCanvas, aOther );
+    wxCHECK( context, nullptr );
+
+    if( !context->IsOK() )
+    {
+        delete context;
+        return nullptr;
+    }
+
+    m_glContexts.insert( std::make_pair( context, aCanvas ) );
+
+    return context;
+}
+
+
+void GL_CONTEXT_MANAGER::DestroyCtx( wxGLContext* aContext )
+{
+    if( m_glContexts.count( aContext ) )
+    {
+        m_glContexts.erase( aContext );
+        delete aContext;
+    }
+    else
+    {
+        // Do not delete unknown GL contexts
+        wxFAIL;
+    }
+
+    if( m_glCtx == aContext )
+        m_glCtx = nullptr;
+}
+
+
+void GL_CONTEXT_MANAGER::DeleteAll()
+{
+    m_glCtxMutex.lock();
+
+    for( auto& ctx : m_glContexts )
+        delete ctx.first;
+
+    m_glContexts.clear();
+    m_glCtx = nullptr;
+    m_glCtxMutex.unlock();
+}
+
+
+void GL_CONTEXT_MANAGER::LockCtx( wxGLContext* aContext, wxGLCanvas* aCanvas )
+{
+    wxCHECK( aContext && m_glContexts.count( aContext ) > 0, /* void */ );
+
+    m_glCtxMutex.lock();
+    wxGLCanvas* canvas = aCanvas ? aCanvas : m_glContexts.at( aContext );
+
+    // Prevent assertion failure in wxGLContext::SetCurrent during GAL teardown
+#ifdef __WXGTK__
+
+#ifdef KICAD_USE_EGL
+    if( canvas->GTKGetDrawingWindow() )
+#else
+    if( canvas->GetXWindow() )
+#endif // KICAD_USE_EGL
+
+#endif // __WXGTK__
+    {
+        canvas->SetCurrent( *aContext );
+    }
+
+    m_glCtx = aContext;
+}
+
+
+void GL_CONTEXT_MANAGER::UnlockCtx( wxGLContext* aContext )
+{
+    wxCHECK( aContext && m_glContexts.count( aContext ) > 0, /* void */ );
+
+    if( m_glCtx == aContext )
+    {
+        m_glCtxMutex.unlock();
+        m_glCtx = nullptr;
+    }
+    else
+    {
+        wxFAIL_MSG( wxString::Format( wxS( "Trying to unlock GL context mutex from "
+                    "a wrong context: aContext %p m_glCtx %p" ), aContext, m_glCtx ) );
+    }
+}
+