Progams code for a certain layer type is encapsulted within
a dedicated `<layer type>Programs` class, inherited from
the generic base `LayerTypePrograms` class.
`mbgl::Programs::get<layer type>Programs()` lazily initializes the
layer type-specific programs code using pointer to the base class,
which allows LTO to remove this code from binaries (if the corresponding
`get<layer type>Programs()` method can never be invoked).
