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/OGL/FramebufferManager.h

@@ -5,8 +5,10 @@
 #pragma once
 
 #include <memory>
+#include <utility>
 #include <vector>
 
+#include "Common/CommonTypes.h"
 #include "Common/GL/GLUtil.h"
 #include "VideoBackends/OGL/ProgramShaderCache.h"
 #include "VideoBackends/OGL/Render.h"
@@ -103,7 +105,7 @@ private:
   std::unique_ptr<XFBSourceBase> CreateXFBSource(unsigned int target_width,
                                                  unsigned int target_height,
                                                  unsigned int layers) override;
-  void GetTargetSize(unsigned int* width, unsigned int* height) override;
+  std::pair<u32, u32> GetTargetSize() const override;
 
   void CopyToRealXFB(u32 xfbAddr, u32 fbStride, u32 fbHeight, const EFBRectangle& sourceRc,
                      float Gamma) override;