Added shared context support

src/Magnum/Platform/WindowlessGlxApplication.cpp

@@ -40,6 +40,15 @@ namespace Magnum { namespace Platform {
 WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configuration& configuration, GLContext* const magnumContext) {
     _display = XOpenDisplay(nullptr);
 
+    /**
+     * The user can choose to create a windowless application with a dedicated OpenGL context
+     * or give an OpenGL context to create a shared Context instead.
+     * 
+     * Here, we ask the configuration whether a context has been specified by the user 
+     * (!= nullptr) or not.
+     */
+    GLXContext ctx = configuration.sharedContext();
+    
     /* Check version */
     int major, minor;
     glXQueryVersion(_display, &major, &minor);
@@ -94,7 +103,7 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
         #endif
         0
     };
-    _context = glXCreateContextAttribsARB(_display, configs[0], nullptr, True, contextAttributes);
+    _context = glXCreateContextAttribsARB(_display, configs[0], ctx, True, contextAttributes);
 
     #ifndef MAGNUM_TARGET_GLES
     /* Fall back to (forward compatible) GL 2.1 if core context creation fails */
@@ -105,7 +114,7 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
             GLX_CONTEXT_FLAGS_ARB, GLint(flags),
             0
         };
-        _context = glXCreateContextAttribsARB(_display, configs[0], nullptr, True, fallbackContextAttributes);
+        _context = glXCreateContextAttribsARB(_display, configs[0], ctx, True, fallbackContextAttributes);
 
     /* Fall back to (forward compatible) GL 2.1 if we are on binary NVidia/AMD
        drivers on Linux. Instead of creating forward-compatible context with
@@ -142,7 +151,7 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
                 GLX_CONTEXT_FLAGS_ARB, GLint(flags & ~Configuration::Flag::ForwardCompatible),
                 0
             };
-            _context = glXCreateContextAttribsARB(_display, configs[0], nullptr, True, fallbackContextAttributes);
+            _context = glXCreateContextAttribsARB(_display, configs[0], ctx, True, fallbackContextAttributes);
         }
 
         /* Revert back the old context */

src/Magnum/Platform/WindowlessGlxApplication.h

@@ -238,8 +238,34 @@ class WindowlessGlxContext::Configuration {
             _flags &= ~flags;
             return *this;
         }
+        
+        /**
+         * @brief Creates an OpenGL shared context with @param ctx instead
+         * of creating a brand new one.
+         */
+        Configuration& setSharedcontext(GLXContext 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
+         */
+        GLXContext sharedContext() const {
+            return _sharedContext;
+        }
 
     private:
+        /**
+         * If the @ref Configuration opengl context
+         * is shared with another context, then _sharedContext points to 
+         * this context.
+         * 
+         * Otherwise = nullptr;
+         */
+        GLXContext _sharedContext{nullptr};
         Flags _flags;
 };