Fix (incorrect) implicit-fallthrough warnings in gcc7

All cases are annotated that it's an explicit fallthrough, so add
Q_FALLTHROUGH to silence the warning.

plugins/kdecorations/aurorae/themes/plastik/code/plastikbutton.cpp

@@ -281,6 +281,7 @@ QPixmap PlastikButtonProvider::icon(ButtonIcon icon, int size, bool active, bool
             drawObject(p, HorizontalLine, r.x()+(r.width()-lwMark)/2, r.y()+(r.height()-lwMark)/2, lwMark, lwMark);
 
             // Fall through to OnAllDesktopsIcon intended!
+            Q_FALLTHROUGH();
         }
         case OnAllDesktopsIcon:
         {
@@ -306,6 +307,7 @@ QPixmap PlastikButtonProvider::icon(ButtonIcon icon, int size, bool active, bool
                 drawObject(p, HorizontalLine, center-(lwArrow-2), r.y()+2*lwArrow, (lwArrow-2)*2, lwArrow);
 
             // Fall through to KeepAboveIcon intended!
+            Q_FALLTHROUGH();
         }
         case KeepAboveIcon:
         {
@@ -331,6 +333,7 @@ QPixmap PlastikButtonProvider::icon(ButtonIcon icon, int size, bool active, bool
                 drawObject(p, HorizontalLine, center-(lwArrow-2), r.bottom()-(lwArrow-1)-2*lwArrow, (lwArrow-2)*2, lwArrow);
 
             // Fall through to KeepBelowIcon intended!
+            Q_FALLTHROUGH();
         }
         case KeepBelowIcon:
         {

plugins/platforms/x11/standalone/x11_platform.cpp

@@ -97,6 +97,7 @@ OpenGLBackend *X11StandalonePlatform::createOpenGLBackend()
         } else {
             qCWarning(KWIN_X11STANDALONE) << "Glx not available, trying EGL instead.";
             // no break, needs fall-through
+            Q_FALLTHROUGH();
         }
 #endif
     case EglPlatformInterface:
@@ -211,6 +212,7 @@ void X11StandalonePlatform::createOpenGLSafePoint(OpenGLSafePoint safePoint)
         group.writeEntry(unsafeKey, true);
         group.sync();
         // Deliberately continue with PreFrame
+        Q_FALLTHROUGH();
     case OpenGLSafePoint::PreFrame:
         if (m_openGLFreezeProtectionThread == nullptr) {
             Q_ASSERT(m_openGLFreezeProtection == nullptr);
@@ -239,6 +241,7 @@ void X11StandalonePlatform::createOpenGLSafePoint(OpenGLSafePoint safePoint)
         group.writeEntry(unsafeKey, false);
         group.sync();
         // Deliberately continue with PostFrame
+        Q_FALLTHROUGH();
     case OpenGLSafePoint::PostFrame:
         QMetaObject::invokeMethod(m_openGLFreezeProtection, "stop", Qt::QueuedConnection);
         break;

scene_opengl.cpp

@@ -525,6 +525,7 @@ void SceneOpenGL::initDebugOutput()
             if (strstr(message, "Buffer detailed info:") && strstr(message, "has been updated"))
                 scheduleVboReInit();
             // fall through! for general message printing
+            Q_FALLTHROUGH();
         case GL_DEBUG_TYPE_DEPRECATED_BEHAVIOR:
         case GL_DEBUG_TYPE_PORTABILITY:
         case GL_DEBUG_TYPE_PERFORMANCE: