FramebufferManagerBase: Return a std::pair from GetTargetSize

Keeps associated data together. It also eliminates the possibility of out
parameters not being initialized properly. For example, consider the
following example:

-- some FramebufferManager implementation --

void FBMgrImpl::GetTargetSize(u32* width, u32* height) override
{
  // Do nothing
}

-- somewhere else where the function is used --

u32 width, height;
framebuffer_manager_instance->GetTargetSize(&width, &height);

if (texture_width != width) <-- Uninitialized variable usage
{
  ...
}

It makes it much more obvious to spot any initialization issues, because
it requires something to be returned, as opposed to allowing an
implementation to just not do anything.

Source/Core/VideoCommon/FramebufferManagerBase.cpp

@@ -3,9 +3,12 @@
 // Refer to the license.txt file included.
 
 #include "VideoCommon/FramebufferManagerBase.h"
+
 #include <algorithm>
 #include <array>
 #include <memory>
+#include <tuple>
+
 #include "VideoCommon/RenderBase.h"
 #include "VideoCommon/VideoConfig.h"
 
@@ -158,8 +161,8 @@ void FramebufferManagerBase::CopyToVirtualXFB(u32 xfbAddr, u32 fbStride, u32 fbH
   if (m_virtualXFBList.begin() != vxfb)
     m_virtualXFBList.splice(m_virtualXFBList.begin(), m_virtualXFBList, vxfb);
 
-  unsigned int target_width, target_height;
-  g_framebuffer_manager->GetTargetSize(&target_width, &target_height);
+  u32 target_width, target_height;
+  std::tie(target_width, target_height) = g_framebuffer_manager->GetTargetSize();
 
   // recreate if needed
   if (vxfb->xfbSource &&