GL: no need to set _viewport twice in Framebuffer constructor.

That line was unchanged since 2012, so I don't think it carried some
meaning that escaped my attention.

src/Magnum/GL/Framebuffer.cpp

@@ -99,7 +99,6 @@ Int Framebuffer::maxColorAttachments() {
 
 Framebuffer::Framebuffer(const Range2Di& viewport): AbstractFramebuffer{0, viewport, ObjectFlag::DeleteOnDestruction} {
     CORRADE_INTERNAL_ASSERT(viewport != Implementation::FramebufferState::DisengagedViewport);
-    _viewport = viewport;
     (this->*Context::current().state().framebuffer.createImplementation)();
     CORRADE_INTERNAL_ASSERT(_id != Implementation::State::DisengagedBinding);
 }