Added WindowlessEGLApplication shared context support

src/Magnum/Platform/WindowlessEglApplication.cpp

@@ -273,7 +273,7 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
     }
     #endif
 
-    if(!(_context = eglCreateContext(_display, config, EGL_NO_CONTEXT, attributes))) {
+    if(!(_context = eglCreateContext(_display, config, configuration.sharedContext(), attributes))) {
         Error() << "Platform::WindowlessEglApplication::tryCreateContext(): cannot create EGL context:" << Implementation::eglErrorString(eglGetError());
         return;
     }

src/Magnum/Platform/WindowlessEglApplication.h

@@ -255,10 +255,28 @@ class WindowlessEglContext::Configuration {
             _device = id;
             return *this;
         }
+        /**
+         * @brief Creates an OpenGL shared context with @param ctx instead
+         * of creating a brand new one.
+         */
+        Configuration& setSharedcontext(EGLContext 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
+         */
+        EGLContext sharedContext() const {
+            return _sharedContext;
+        }       
         #endif
 
     private:
         #ifndef MAGNUM_TARGET_WEBGL
+        EGLContext _sharedContext{EGL_NO_CONTEXT};
         Flags _flags;
         UnsignedInt _device;
         #endif