Remove empty record types

SDL's API contains a lot of opaque data types, where the user is given
a pointer and is expected to never dereference it. Until now, we used
empty records to denote such types, and then defined pointer types to
said empty records. This commit removes all(*) the opaque types and
instead declares pointer types using the "type Pointer" notation.
This makes it clear that the user is expected not to use the "raw"
types, only pointer types, while maintaining type safety.

Check the discussion in the link below for more details
about the rationale behind this change.
#63

(*) units/sdl2_net.pas still contains some empty records.
However, that file is very messy right now and needs a proper rewrite.

Author: suve

units/sdl2_mixer.pas

@@ -166,8 +166,7 @@   TMix_Chunk = record
 
   {* The internal format for a music chunk interpreted via mikmod *}
   PPMix_Music = ^PMix_Music;
-  PMix_Music = ^TMix_Music;
-  TMix_Music = record end;
+  PMix_Music = type Pointer;
 
   {* Open the mixer with a certain audio format *}
 function Mix_OpenAudio(frequency: cint; format: cuint16; channels: cint; chunksize: cint): cint cdecl; external MIX_LibName {$IFDEF DELPHI} {$IFDEF MACOS} name '_MIX_OpenAudio' {$ENDIF} {$ENDIF};
@@ -241,7 +240,7 @@ function Mix_HasMusicDecoder(const name: PAnsiChar): TSDL_Bool cdecl;
   {* Find out the music format of a mixer music, or the currently playing
      music, if 'music' is NULL.
   *}
-function Mix_GetMusicType(music: TMix_Music): TMix_MusicType cdecl; external MIX_LibName {$IFDEF DELPHI} {$IFDEF MACOS} name '_MIX_GetMusicType' {$ENDIF} {$ENDIF};
+function Mix_GetMusicType(music: PMix_Music): TMix_MusicType cdecl; external MIX_LibName {$IFDEF DELPHI} {$IFDEF MACOS} name '_MIX_GetMusicType' {$ENDIF} {$ENDIF};
 
   {* Set a function that is called after all mixing is performed.
      This can be used to provide real-time visual display of the audio stream

units/sdl2_ttf.pas

@@ -175,8 +175,7 @@ procedure TTF_ByteSwappedUNICODE(swapped: TSDL_bool); cdecl;
 {* The internal structure containing font information *}
 type
   PPTTF_Font = ^PTTF_Font;
-  PTTF_Font = ^TTTF_Font;
-  TTTF_Font = record  end; //todo?
+  PTTF_Font = type Pointer;
 
 {*
  * Initialize SDL_ttf.

units/sdlaudio.inc

@@ -942,8 +942,7 @@ function SDL_ConvertAudio(cvt: PSDL_AudioCVT): cint; cdecl;
   }
 { this is opaque to the outside world.  }
 type
-  PSDL_AudioStream = ^TSDL_AudioStream;
-  TSDL_AudioStream = record end;
+  PSDL_AudioStream = type Pointer;
 
 {*
  * Create a new audio stream.

units/sdlgamecontroller.inc

@@ -15,8 +15,7 @@
 {* The gamecontroller structure used to identify an SDL game controller *}
 type
   PPSDL_GameController = ^PSDL_GameController;
-  PSDL_GameController = ^TSDL_GameController;
-  TSDL_GameController = record end;
+  PSDL_GameController = type Pointer;
 
   PPSDL_GameControllerType = ^PSDL_GameControllerType;
   PSDL_GameControllerType = ^TSDL_GameControllerType;

units/sdlhaptic.inc

@@ -93,8 +93,7 @@
    *}
 type
   PPSDL_Haptic = ^PSDL_Haptic;
-  PSDL_Haptic = ^TSDL_Haptic;
-  TSDL_Haptic = record end;
+  PSDL_Haptic = type Pointer;
 
   {**
    *   Haptic features

units/sdlhidapi.inc

@@ -42,8 +42,7 @@ type
 	(**
  	 *  \brief  A handle representing an open HID device.
  	 *)
-	PSDL_hid_device = ^TSDL_hid_device;
-	TSDL_hid_device = record end; // opaque struct
+	PSDL_hid_device = type Pointer;
 
 	PSDL_hid_device_info = ^TSDL_hid_device_info;
 

units/sdljoystick.inc

@@ -30,8 +30,7 @@
 type
   {* The joystick structure used to identify an SDL joystick *}
   PPSDL_Joystick = ^PSDL_Joystick;
-  PSDL_Joystick = ^TSDL_Joystick;
-  TSDL_Joystick = record end;
+  PSDL_Joystick = type Pointer;
 
   {* A structure that encodes the stable unique id for a joystick device *}
   PPSDL_JoystickGUID = ^PSDL_JoystickGUID;

units/sdlrenderer.inc

@@ -91,17 +91,13 @@ type
    *}
 
   PPSDL_Renderer = ^PSDL_Renderer;
-  PSDL_Renderer = ^TSDL_Renderer;
-  TSDL_Renderer = record
-    end;
+  PSDL_Renderer = type Pointer;
 
   {**
    *  An efficient driver-specific representation of pixel data
    *}
   PPSDL_Texture = ^PSDL_Texture;
-  PSDL_Texture = ^TSDL_Texture;
-  TSDL_Texture = record
-    end;
+  PSDL_Texture = type Pointer;
 
   {* Function prototypes *}
 

units/sdlsensor.inc

@@ -10,8 +10,7 @@
  *}
 type
 	PPSDL_Sensor = ^PSDL_Sensor;
-	PSDL_Sensor = ^TSDL_Sensor;
-	TSDL_Sensor = record end;
+	PSDL_Sensor = type Pointer;
 
 {**
  * This is a unique ID for a sensor for the time it is connected to the system,

units/sdlstdinc.inc

@@ -685,8 +685,7 @@ const
   SDL_ICONV_EINVAL = csize_t(-4);
 
 type
-  TSDL_iconv = record end;
-  PSDL_iconv = ^TSDL_iconv;
+  PSDL_iconv = type Pointer;
 
 function SDL_iconv_open(Const tocode, fromcode: PAnsiChar): PSDL_iconv; cdecl;
   external SDL_LibName {$IFDEF DELPHI} {$IFDEF MACOS} name '_SDL_iconv_open' {$ENDIF} {$ENDIF};