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/Vulkan/FramebufferManager.cpp

@@ -115,10 +115,9 @@ bool FramebufferManager::Initialize()
   return true;
 }
 
-void FramebufferManager::GetTargetSize(unsigned int* width, unsigned int* height)
+std::pair<u32, u32> FramebufferManager::GetTargetSize() const
 {
-  *width = m_efb_width;
-  *height = m_efb_height;
+  return std::make_pair(m_efb_width, m_efb_height);
 }
 
 bool FramebufferManager::CreateEFBRenderPass()