multimedia/aegisub: simplify + unbreak boost 1.62 on 9.x

In file included from libaegisub/ass/time.cpp:20:0:
libaegisub/include/libaegisub/format.h: In static member function 'static Out agi::format_detail::runtime_cast_helper<In, Out, <anonymous> >::cast(const In&)':
libaegisub/include/libaegisub/format.h:31:37: error: 'bad_cast' is not a member of 'std'

static Out cast(In const&) { throw std::bad_cast(); }
                                   ^

In file included from libaegisub/lua/script_reader.cpp:19:0:
libaegisub/include/libaegisub/file_mapping.h:37:3: error: 'unique_ptr' in namespace 'std' does not name a type

std::unique_ptr<boost::interprocess::mapped_region> region;
^

libaegisub/include/libaegisub/file_mapping.h:54:3: error: 'unique_ptr' in namespace 'std' does not name a type

std::unique_ptr<boost::interprocess::mapped_region> read_region;
^

libaegisub/include/libaegisub/file_mapping.h:56:3: error: 'unique_ptr' in namespace 'std' does not name a type

std::unique_ptr<boost::interprocess::mapped_region> write_region;
^

PR: 199601