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/VideoBackends/D3D/FramebufferManager.cpp

@@ -2,13 +2,15 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#include "VideoBackends/D3D/FramebufferManager.h"
+
 #include <memory>
 
+#include "Common/CommonTypes.h"
 #include "Core/HW/Memmap.h"
 #include "VideoBackends/D3D/D3DBase.h"
 #include "VideoBackends/D3D/D3DState.h"
 #include "VideoBackends/D3D/D3DUtil.h"
-#include "VideoBackends/D3D/FramebufferManager.h"
 #include "VideoBackends/D3D/GeometryShaderCache.h"
 #include "VideoBackends/D3D/PixelShaderCache.h"
 #include "VideoBackends/D3D/Render.h"
@@ -292,10 +294,9 @@ std::unique_ptr<XFBSourceBase> FramebufferManager::CreateXFBSource(unsigned int
       layers);
 }
 
-void FramebufferManager::GetTargetSize(unsigned int* width, unsigned int* height)
+std::pair<u32, u32> FramebufferManager::GetTargetSize() const
 {
-  *width = m_target_width;
-  *height = m_target_height;
+  return std::make_pair(m_target_width, m_target_height);
 }
 
 void XFBSource::DecodeToTexture(u32 xfbAddr, u32 fbWidth, u32 fbHeight)