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.h

@@ -7,6 +7,7 @@
 #include <array>
 #include <list>
 #include <memory>
+#include <utility>
 
 #include "Common/CommonTypes.h"
 #include "VideoCommon/VideoCommon.h"
@@ -60,7 +61,7 @@ public:
   static int ScaleToVirtualXfbHeight(int y);
 
   static unsigned int GetEFBLayers() { return m_EFBLayers; }
-  virtual void GetTargetSize(unsigned int* width, unsigned int* height) = 0;
+  virtual std::pair<u32, u32> GetTargetSize() const = 0;
 
 protected:
   struct VirtualXFB