iOS port initial commit

Known Issues
------------
- No printing support.
- No typing support using the OS virtual keyboard.  iOS needs to be signalled
  to bring up the virtual keyboard when the text tool is active.  We also may
  need to do some finagling to make IM work with the virtual keyboard.
- OS language detection doesn't work yet.
- Quitting doesn't close the app.  It just displays a black screen until it is
  force-closed.
- Need to include cross-compilation instructions.

Possible Issues
---------------
- No text display.  This is likely an issue with how pango and related
  libraries were cross-compiled rather than an issue with Tux Paint code.  From
  the error output it appears to be a font rendering issue.
- SVG integration couldn't be tested because RSVG library has not yet be
  cross-compiled successfully.
- Only tested under the iOS Simulator (and not on an actual iOS device yet.)

src/tuxpaint.c

@@ -25,6 +25,7 @@
   June 14, 2002 - March 8, 2021
 */
 
+#include "platform.h"
 
 /* (Note: VER_VERSION and VER_DATE are now handled by Makefile) */
 
@@ -303,15 +304,21 @@ typedef struct safer_dirent
 
 /* Not BeOS */
 
-#ifdef __APPLE__
+#if defined(__MACOS__)
 
-/* Apple */
+/* macOS */
 
 #include "macos_print.h"
 
-#else /* __APPLE__ */
+#elif defined(__IOS__)
 
-/* Not Windows, not BeOS, not Apple */
+/* iOS */
+
+#include "ios_print.h"
+
+#else /* __MACOS__, __IOS__ */
+
+/* Not Windows, not BeOS, not macOS, not iOS */
 #ifdef __ANDROID__
 
 #define AUTOSAVE_GOING_BACKGROUND
@@ -326,7 +333,7 @@ typedef struct safer_dirent
 
 #endif /* __ANDROID__ */
 
-#endif /* __APPLE__ */
+#endif /* __MACOS__, __IOS__ */
 
 #endif /* __BEOS__ */
 
@@ -373,8 +380,10 @@ static void mtw(wchar_t * wtok, char *tok)
 
 #endif /* WIN32 */
 
-#ifdef __APPLE__
+#if defined(__MACOS__)
 #include "macos.h"
+#elif defined(__IOS__)
+#include "ios.h"
 #endif
 
 #include <errno.h>
@@ -17718,7 +17727,7 @@ void do_print(void)
 
   SurfacePrint(save_canvas);
 #elif defined(__APPLE__)
-  /* Mac OS X */
+  /* macOS, iOS */
   int show = (want_alt_printcommand && !fullscreen);
 
   const char *error = SurfacePrint(save_canvas, show);
@@ -23925,7 +23934,7 @@ static void setup_config(char *argv[])
       result = find_directory(B_USER_SETTINGS_DIRECTORY, volume, false, buffer, sizeof(buffer));
       asprintf((char **)&savedir, "%s/%s", buffer, "TuxPaint");
 #elif __APPLE__
-      savedir = strdup(macos_preferencesPath());
+      savedir = strdup(apple_preferencesPath());
 #elif __ANDROID__
       savedir = SDL_AndroidGetExternalStoragePath();
 #else
@@ -23969,8 +23978,8 @@ static void setup_config(char *argv[])
   /* BeOS: Use a "tuxpaint.cfg" file: */
   strcpy(str, "tuxpaint.cfg"); /* safe; sufficient size */
 #elif defined(__APPLE__)
-  /* Mac OS X: Use a "tuxpaint.cfg" file in the Tux Paint application support folder */
-  safe_snprintf(str, sizeof(str), "%s/tuxpaint.cfg", macos_preferencesPath());
+  /* macOS, iOS: Use a "tuxpaint.cfg" file in the Tux Paint application support folder */
+  safe_snprintf(str, sizeof(str), "%s/tuxpaint.cfg", apple_preferencesPath());
 #elif defined(__ANDROID__)
   /* Try to find the user's config file */
   /* This file is writed by the tuxpaint config activity when the user runs it */
@@ -24004,9 +24013,9 @@ static void setup_config(char *argv[])
 #elif defined(__APPLE__)
       /* EP added this conditional section for Mac to fix
          folder & extension inconsistency with Tux Paint Config application) */
-      /* Mac OS X: Use a "tuxpaint.cfg" file in the *global* Tux Paint
+      /* macOS, iOS: Use a "tuxpaint.cfg" file in the *global* Tux Paint
          application support folder */
-      safe_snprintf(str, sizeof(str), "%s/tuxpaint.cfg", macos_globalPreferencesPath());
+      safe_snprintf(str, sizeof(str), "%s/tuxpaint.cfg", apple_globalPreferencesPath());
       parse_file_options(&tmpcfg_sys, str);
 #elif defined(__ANDROID__)
       /* Load the config file we provide in assets/etc/tuxpaint.cfg */
@@ -24500,7 +24509,7 @@ static void chdir_to_binary(char *argv0)
       char *app_path = strdup(argv0);
       char *slash = strrchr(app_path, '/');
 
-#if defined(__APPLE__)
+#if defined(__MACOS__)
       // On macOS, execution is deep inside the app bundle.
       // E.g., "/Applications/TuxPaint.app/Contents/MacOS/tuxpaint"
       // But we want to point somewhere higher up, say to "Contents", so we can access