Added sharedContextSupport for CGL and WGL windowlessApplication

src/Magnum/Platform/WindowlessCglApplication.cpp

@@ -35,7 +35,7 @@
 
 namespace Magnum { namespace Platform {
 
-WindowlessCglContext::WindowlessCglContext(const Configuration&, GLContext*) {
+WindowlessCglContext::WindowlessCglContext(const Configuration & configuration, GLContext*) {
     int formatCount;
     CGLPixelFormatAttribute attributes32[] = {
         kCGLPFAAccelerated,
@@ -68,7 +68,7 @@ WindowlessCglContext::WindowlessCglContext(const Configuration&, GLContext*) {
         }
     }
 
-    if(CGLCreateContext(_pixelFormat, nullptr, &_context) != kCGLNoError)
+    if(CGLCreateContext(_pixelFormat, configuration.sharedContext(), &_context) != kCGLNoError)
         Error() << "Platform::WindowlessCglContext: cannot create context";
 }
 

src/Magnum/Platform/WindowlessCglApplication.h

@@ -142,6 +142,28 @@ class WindowlessCglContext {
 class WindowlessCglContext::Configuration {
     public:
         constexpr /*implicit*/ Configuration() {}
+        
+        /**
+         * @brief Creates an OpenGL shared context with @param ctx instead
+         * of creating a brand new one.
+         */
+        Configuration& setSharedcontext(CGLContextObj ctx) {
+            _sharedContext = ctx;
+            return *this;
+        }
+        
+        /**
+         * @brief Returns the configuration shared context.
+         * If this has not been specified, (meaning the configuration uses a new opengl context), 
+         * then returns nullptr
+         */
+        CGLContextObj sharedContext() const {
+            return _sharedContext;
+        }
+        
+private:
+    CGLContextObj _context{};
+    CGLContextObj _sharedContext{};
 };
 
 /**

src/Magnum/Platform/WindowlessWglApplication.cpp

@@ -145,7 +145,7 @@ WindowlessWglContext::WindowlessWglContext(const Configuration& configuration, G
         #endif
         0
     };
-    _context = wglCreateContextAttribsARB(_deviceContext, nullptr, contextAttributes);
+    _context = wglCreateContextAttribsARB(_deviceContext, configuration.sharedContext(), contextAttributes);
 
     #ifndef MAGNUM_TARGET_GLES
     /* Fall back to (forward compatible) GL 2.1 if core context creation fails */
@@ -157,7 +157,7 @@ WindowlessWglContext::WindowlessWglContext(const Configuration& configuration, G
             WGL_CONTEXT_FLAGS_ARB, GLint(flags & ~Configuration::Flag::ForwardCompatible),
             0
         };
-        _context = wglCreateContextAttribsARB(_deviceContext, nullptr, fallbackContextAttributes);
+        _context = wglCreateContextAttribsARB(_deviceContext, configuration.sharedContext(), fallbackContextAttributes);
 
     /* Fall back to (forward compatible) GL 2.1 if we are on binary
        NVidia/AMD/Intel drivers on Windows. Instead of creating forward-compatible
@@ -196,7 +196,7 @@ WindowlessWglContext::WindowlessWglContext(const Configuration& configuration, G
                 WGL_CONTEXT_FLAGS_ARB, GLint(flags & ~Configuration::Flag::ForwardCompatible),
                 0
             };
-            _context = wglCreateContextAttribsARB(_deviceContext, nullptr, fallbackContextAttributes);
+            _context = wglCreateContextAttribsARB(_deviceContext, configuration.sharedContext(), fallbackContextAttributes);
         }
     }
     #endif

src/Magnum/Platform/WindowlessWglApplication.h

@@ -232,9 +232,29 @@ class WindowlessWglContext::Configuration {
             _flags &= ~flags;
             return *this;
         }
+        
+                /**
+         * @brief Creates an OpenGL shared context with @param ctx instead
+         * of creating a brand new one.
+         */
+        Configuration& setSharedcontext(HGLRC ctx) {
+            _sharedContext = ctx;
+            return *this;
+        }
+        
+        /**
+         * @brief Returns the configuration shared context.
+         * If this has not been specified, (meaning the configuration uses a new opengl context), 
+         * then returns nullptr
+         */
+        HGLRC sharedContext() const {
+            return _sharedContext;
+        }
 
     private:
+        HGLRC _sharedContext;
         Flags _flags;
+        
 };
 
 CORRADE_ENUMSET_OPERATORS(WindowlessWglContext::Configuration::Flags)