Parametrize disengaged viewport instead of using default-constructed value.

src/Magnum/AbstractFramebuffer.cpp

@@ -199,6 +199,7 @@ void AbstractFramebuffer::blitImplementationNV(AbstractFramebuffer& source, Abst
 #endif
 
 AbstractFramebuffer& AbstractFramebuffer::setViewport(const Range2Di& rectangle) {
+    CORRADE_INTERNAL_ASSERT(rectangle != Implementation::FramebufferState::DisengagedViewport);
     _viewport = rectangle;
 
     /* Update the viewport if the framebuffer is currently bound */
@@ -211,8 +212,7 @@ AbstractFramebuffer& AbstractFramebuffer::setViewport(const Range2Di& rectangle)
 void AbstractFramebuffer::setViewportInternal() {
     Implementation::FramebufferState& state = *Context::current()->state().framebuffer;
 
-    /* We are using empty viewport to indicate disengaged state */
-    CORRADE_INTERNAL_ASSERT(_viewport != Range2Di{});
+    CORRADE_INTERNAL_ASSERT(_viewport != Implementation::FramebufferState::DisengagedViewport);
     CORRADE_INTERNAL_ASSERT(state.drawBinding == _id);
 
     /* Already up-to-date, nothing to do */

src/Magnum/DefaultFramebuffer.cpp

@@ -101,6 +101,7 @@ void DefaultFramebuffer::initializeContextBasedFunctionality(Context& context) {
     GLint viewport[4];
     glGetIntegerv(GL_VIEWPORT, viewport);
     defaultFramebuffer._viewport = state.viewport = Range2Di::fromSize({viewport[0], viewport[1]}, {viewport[2], viewport[3]});
+    CORRADE_INTERNAL_ASSERT(defaultFramebuffer._viewport != Implementation::FramebufferState::DisengagedViewport);
 
     /* Fake initial glViewport() call for ApiTrace */
     #ifndef MAGNUM_TARGET_GLES

src/Magnum/Framebuffer.cpp

@@ -80,6 +80,7 @@ Int Framebuffer::maxColorAttachments() {
 }
 
 Framebuffer::Framebuffer(const Range2Di& viewport) {
+    CORRADE_INTERNAL_ASSERT(viewport != Implementation::FramebufferState::DisengagedViewport);
     _viewport = viewport;
     (this->*Context::current()->state().framebuffer->createImplementation)();
     CORRADE_INTERNAL_ASSERT(_id != Implementation::State::DisengagedBinding);

src/Magnum/Implementation/FramebufferState.cpp

@@ -33,10 +33,13 @@
 
 namespace Magnum { namespace Implementation {
 
-FramebufferState::FramebufferState(Context& context, std::vector<std::string>& extensions): readBinding(0), drawBinding(0), renderbufferBinding(0), maxDrawBuffers(0), maxColorAttachments(0), maxRenderbufferSize(0), maxSamples(0)
+constexpr const Range2Di FramebufferState::DisengagedViewport;
+
+FramebufferState::FramebufferState(Context& context, std::vector<std::string>& extensions): readBinding{0}, drawBinding{0}, renderbufferBinding{0}, maxDrawBuffers{0}, maxColorAttachments{0}, maxRenderbufferSize{0}, maxSamples{0},
     #ifndef MAGNUM_TARGET_GLES
-    , maxDualSourceDrawBuffers(0)
+    maxDualSourceDrawBuffers{0},
     #endif
+    viewport{DisengagedViewport}
 {
     /* Create implementation */
     #ifndef MAGNUM_TARGET_GLES
@@ -236,7 +239,7 @@ FramebufferState::FramebufferState(Context& context, std::vector<std::string>& e
 
 void FramebufferState::reset() {
     readBinding = drawBinding = renderbufferBinding = State::DisengagedBinding;
-    viewport = {};
+    viewport = DisengagedViewport;
 }
 
 }}

src/Magnum/Implementation/FramebufferState.h

@@ -33,6 +33,8 @@
 namespace Magnum { namespace Implementation {
 
 struct FramebufferState {
+    constexpr static const Range2Di DisengagedViewport{};
+
     explicit FramebufferState(Context& context, std::vector<std::string>& extensions);
 
     void reset();