https://gitlab.kitware.com/ben.boeckel/paraview/commit/40a62e0bd20ceb73d7a545dab73e975fa07cddbb
http://www.vtk.org/Bug/view.php?id=15442#c34997

https://bugs.gentoo.org/show_bug.cgi?id=537988

This updates the *.proto files to be compatible with version 2.6 and fixes
a bug that prevented successful compilation with protobuf 2.6.

--- a/ParaViewCore/ServerImplementation/Core/vtkPVMessage.proto
+++ b/ParaViewCore/ServerImplementation/Core/vtkPVMessage.proto
@@ -21,7 +21,7 @@ message Variant {
     MAP     = 7;
     }
 
-  required Type    type            = 1;
+  optional Type    type            = 1;
   repeated sint64  idtype          = 3;
   repeated sint32  integer         = 4;
   repeated double  float64         = 5;
@@ -47,8 +47,8 @@ message MessageCollection
 
 message Message
 {
-  required uint64 global_id  = 1;
-  required uint32 location   = 2;
+  optional uint64 global_id  = 1;
+  optional uint32 location   = 2;
   optional bool   share_only = 3 [default = false];
   optional bool   req_def    = 4 [default = false];
   optional uint32 client_id  = 5 [default = 0];
@@ -62,10 +62,10 @@ message DefinitionHeader
 {
   extend Message {
     // The classname for the driver process i.e. the client, typically
-    required string client_class = 10;
+    optional string client_class = 10;
 
     // The classname for the process where the processing occurs
-    required string server_class = 11;
+    optional string server_class = 11;
   }
 }
 
@@ -76,32 +76,32 @@ message ProxyState
 
   message SubProxy
     {
-    required string name      = 1;
-    required uint32 global_id = 2;
+    optional string name      = 1;
+    optional uint32 global_id = 2;
     }
 
   message Annotation
     {
-    required string key   = 1;
-    required string value = 2;
+    optional string key   = 1;
+    optional string value = 2;
     }
 
   message UserData
     {
-    required string key   = 1;
+    optional string key   = 1;
     repeated Variant variant = 2;
     }
 
   message Property
     {
-    required string  name  = 1;
+    optional string  name  = 1;
     optional Variant value = 2;
     repeated UserData user_data = 3;
     }
 
   extend Message {
-    required string     xml_group           = 20;
-    required string     xml_name            = 21;
+    optional string     xml_group           = 20;
+    optional string     xml_name            = 21;
     optional string     xml_sub_proxy_name  = 22;
     repeated Property   property            = 23;
     repeated SubProxy   subproxy            = 24;
@@ -118,8 +118,8 @@ message PXMRegistrationState
   message Entry
     {
     optional string group     = 1;
-    required string name      = 2;
-    required uint64 global_id = 3;
+    optional string name      = 2;
+    optional uint64 global_id = 3;
     }
 
   extend Message {
@@ -135,9 +135,9 @@ message ProxyDefinitionState
 {
   message ProxyXMLDefinition
     {
-    required string group = 1;
-    required string name  = 2;
-    required string xml   = 3;
+    optional string group = 1;
+    optional string name  = 2;
+    optional string xml   = 3;
     }
 
   extend Message {
@@ -181,9 +181,9 @@ message LinkState {
       OUTPUT = 2;
       }
 
-    required uint32 proxy         = 1;
+    optional uint32 proxy         = 1;
     optional string property_name = 2;
-    required Direction direction  = 3;
+    optional Direction direction  = 3;
     }
 
   extend Message {
@@ -206,7 +206,7 @@ message MousePointer {
   }
 
   extend Message {
-    required uint32 view      = 70;
+    optional uint32 view      = 70;
     optional double x         = 71 [default = 0];
     optional double y         = 72 [default = 0];
     optional bool   forceShow = 73 [default = false];
@@ -219,7 +219,7 @@ message MousePointer {
 message ChartViewBounds {
 
   extend Message {
-    required uint32 view  = 75;
+    optional uint32 view  = 75;
     repeated double range = 76;
   }
 }
@@ -229,7 +229,7 @@ message ChartViewBounds {
 message ClientsInformation {
 
   message ClientInfo {
-    required uint32 user       = 1;
+    optional uint32 user       = 1;
     optional string name       = 2;
     optional bool   is_master  = 3 [default = false];
     optional bool   follow_cam = 4 [default = false];
@@ -242,8 +242,8 @@ message ClientsInformation {
 
 message ChatMessage {
   extend Message {
-    required uint32 author = 84;
-    required string txt    = 85;
+    optional uint32 author = 84;
+    optional string txt    = 85;
   }
 }
 
@@ -256,7 +256,7 @@ message QtEvent {
   }
 
   extend Message {
-    required QtEventType type  = 98;
+    optional QtEventType type  = 98;
   }
 }
 
--- a/ParaViewCore/ServerManager/Rendering/vtkSMCameraLink.cxx
+++ b/ParaViewCore/ServerManager/Rendering/vtkSMCameraLink.cxx
@@ -363,6 +363,6 @@ void vtkSMCameraLink::LoadState(const vtkSMMessage *msg, vtkSMProxyLocator *loca
 void vtkSMCameraLink::UpdateState()
 {
   this->Superclass::UpdateState();
-  this->State->GetExtension(LinkState::sync_interactive_renders,
+  this->State->SetExtension(LinkState::sync_interactive_renders,
     this->GetSynchronizeInteractiveRenders());
 }