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();

Source/Core/VideoCommon/PixelShaderManager.cpp

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

Source/Core/VideoCommon/VertexShaderManager.cpp

@@ -203,8 +203,8 @@ void VertexShaderManager::Init()
   bProjectionChanged = true;
   bViewportChanged = false;
 
-  memset(&xfmem, 0, sizeof(xfmem));
-  memset(&constants, 0, sizeof(constants));
+  xfmem = {};
+  constants = {};
   ResetView();
 
   // TODO: should these go inside ResetView()?