VideoCommon: Use std::span for BoundingBox::Write()

Crosses off a lingering TODO.

Also amends a few nearby cases where a u32 cast was being repromoted to
size_t.

Source/Core/VideoBackends/Null/NullBoundingBox.h

@@ -19,7 +19,7 @@ protected:
   {
     return std::vector<BBoxType>(length);
   }
-  void Write(u32 index, const std::vector<BBoxType>& values) override {}
+  void Write(u32 index, std::span<const BBoxType> values) override {}
 };
 
 }  // namespace Null