GL: Add Context flag for disabling framebuffer initialization

src/Magnum/GL/Context.cpp

@@ -912,7 +912,9 @@ bool Context::tryCreate() {
 
     /* Initialize functionality based on current OpenGL version and extensions */
     /** @todo Get rid of these */
-    DefaultFramebuffer::initializeContextBasedFunctionality(*this);
+    if(!(_internalFlags & InternalFlag::NoFramebuffer))
+        DefaultFramebuffer::initializeContextBasedFunctionality(*this);
+
     Renderer::initializeContextBasedFunctionality();
 
     /* Enable GPU validation, if requested */

src/Magnum/GL/Context.h

@@ -747,6 +747,8 @@ class MAGNUM_GL_EXPORT Context {
             DisplayVerboseInitializationLog = DisplayInitializationLog|(1 << 1),
             GpuValidation = 1 << 2,
 
+            NoFramebuffer = 1 << 3,
+
             Default = DisplayInitializationLog
         };
         typedef Containers::EnumSet<InternalFlag> InternalFlags;