Allow loading CSS17.DAT and MP.DAT on all three RCT1 sources

[Gymnasiast]

This should equalise the three in terms of supported features.

[AaronVanGeffen]

Unfortunately, this PR has caused a regression:

$ gdb ./openrct2
[...]
terminate called after throwing an instance of 'IOException'
  what():  Unable to open ''

Thread 1 "openrct2" received signal SIGABRT, Aborted.
0x00007ffff5a21b5f in raise () from /usr/lib/libc.so.6
(gdb) bt
#0  0x00007ffff5a21b5f in raise () from /usr/lib/libc.so.6
#1  0x00007ffff5a0c452 in abort () from /usr/lib/libc.so.6
#2  0x00007ffff5ddf58e in __gnu_cxx::__verbose_terminate_handler () at /build/gcc/src/gcc/libstdc++-v3/libsupc++/vterminate.cc:95
#3  0x00007ffff5de5dfa in __cxxabiv1::__terminate(void (*)()) () at /build/gcc/src/gcc/libstdc++-v3/libsupc++/eh_terminate.cc:47
#4  0x00007ffff5de5e57 in std::terminate () at /build/gcc/src/gcc/libstdc++-v3/libsupc++/eh_terminate.cc:57
#5  0x00007ffff5de60ac in __cxxabiv1::__cxa_throw (obj=<optimized out>, tinfo=0x5555561f9a90 <typeinfo for IOException>, 
    dest=0x55555581bd20 <IOException::~IOException()>) at /build/gcc/src/gcc/libstdc++-v3/libsupc++/eh_throw.cc:95
#6  0x000055555581c1b7 in FileStream::FileStream(char const*, int) ()
#7  0x000055555581bcb6 in FileStream::FileStream(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int) ()
#8  0x00005555559402ea in File::WriteAllBytes(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, void const*, unsigned long) ()
#9  0x0000555555eae564 in ScenarioRepository::ConvertMegaPark(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ()
#10 0x0000555555ead8ea in ScenarioRepository::ImportMegaPark() ()
#11 0x0000555555eaaf6b in ScenarioRepository::Scan(int) ()
#12 0x00005555558f5e4d in OpenRCT2::Context::Initialise() ()
#13 0x00005555558f5523 in OpenRCT2::Context::RunOpenRCT2(int, char const**) ()
#14 0x00005555557edd27 in main ()