Use inheritance to remove This data member

It's reasonable to say that, for example, sfImage _is_ an sf::Image.
This "is-a" relationship is what inheritance does a very good job
modeling. By using inheritance to implement CSFML types in terms of
their SFML counterpart, we get code that is simpler and easier to
read. We also avoid some of the extra "dancing" required to construct
a CSFML type from an SFML type. This improves the encapsulation of
our CSFML implementation.

.clang-tidy

@@ -15,6 +15,7 @@ Checks: >
   -cppcoreguidelines-macro-usage,
   -cppcoreguidelines-narrowing-conversions,
   -cppcoreguidelines-no-malloc,
+  -cppcoreguidelines-non-private-member-variables-in-classes,
   -cppcoreguidelines-owning-memory,
   -cppcoreguidelines-pro-bounds-array-to-pointer-decay,
   -cppcoreguidelines-pro-bounds-constant-array-index,