Emit fake glViewport() call for the first time if ApiTrace is detected.

src/DefaultFramebuffer.cpp

@@ -72,6 +72,12 @@ void DefaultFramebuffer::initializeContextBasedFunctionality(Context* context) {
     GLint viewport[4];
     glGetIntegerv(GL_VIEWPORT, viewport);
     defaultFramebuffer._viewport = state->viewport = Rectanglei::fromSize({viewport[0], viewport[1]}, {viewport[2], viewport[3]});
+
+    /* Fake initial glViewport() call for ApiTrace */
+    #ifndef MAGNUM_TARGET_GLES
+    if(context->isExtensionSupported<Extensions::GL::GREMEDY::string_marker>())
+        glViewport(viewport[0], viewport[1], viewport[2], viewport[3]);
+    #endif
 }
 
 }