ShaderManagers: Use aggregate initialization for some variables.

These provide the same semantics, however aggregate initialization
doesn't force the structs to be trivially copyable. memset, on the other
hand, does.

Source/Core/VideoCommon/GeometryShaderManager.cpp

@@ -21,7 +21,7 @@ static bool s_viewport_changed;
 
 void GeometryShaderManager::Init()
 {
-  memset(&constants, 0, sizeof(constants));
+  constants = {};
 
   // Init any intial constants which aren't zero when bpmem is zero.
   SetViewportChanged();