I wonder,

in sdlstd.inc we define all the types used in SDL2. We lead them back to basic Pascal types. In this example we lead UInt16 back to Word.

PUInt16 = ^UInt16;
UInt16 = Word;

I suggest to have a conditional type conversion bound to the ctypes unit for Free Pascal.

{$IFDEF FPC}
  UInt16 = cuint16;
{$ELSE}
  UInt16 = Word;
{$ENDIF}

The advantage is that type sizes should be chosen even more reliably for different platforms (at least in case of FPC).