frontend: Honor the LIBRETRO_ASSETS_DIRECTORY environment variable.

This builds on 763fcd8 ("unix, win32: Allow set the default
libretro_directory via environment variable") to also allow specifying
the assets directory via an environment variable.

* frontend/drivers/platform_unix.c (frontend_unix_get_env)
<libretro_assets_directory> New variable. Use it to set
DEFAULT_DIR_ASSETS, when available.
* frontend/drivers/platform_win32.c (frontend_win32_env_get): Likewise.

frontend/drivers/platform_unix.c

@@ -1324,6 +1324,7 @@ static void frontend_unix_get_env(int *argc,
 {
    unsigned i;
    const char* libretro_directory = getenv("LIBRETRO_DIRECTORY");
+   const char* libretro_assets_directory = getenv("LIBRETRO_ASSETS_DIRECTORY");
 #ifdef ANDROID
    int32_t major, minor, rel;
    char device_model[PROP_VALUE_MAX]  = {0};
@@ -1778,7 +1779,10 @@ static void frontend_unix_get_env(int *argc,
             "assets", sizeof(g_defaults.dirs[DEFAULT_DIR_ASSETS]));
    else
 #endif
-   if (path_is_directory("/usr/local/share/retroarch/assets"))
+   if (!string_is_empty(libretro_assets_directory))
+      strlcpy(g_defaults.dirs[DEFAULT_DIR_ASSETS], libretro_assets_directory,
+	      sizeof(g_defaults.dirs[DEFAULT_DIR_ASSETS]));
+   else if (path_is_directory("/usr/local/share/retroarch/assets"))
       fill_pathname_join(g_defaults.dirs[DEFAULT_DIR_ASSETS],
             "/usr/local/share/retroarch",
             "assets", sizeof(g_defaults.dirs[DEFAULT_DIR_ASSETS]));

frontend/drivers/platform_win32.c

@@ -565,14 +565,20 @@ static void frontend_win32_env_get(int *argc, char *argv[],
 {
    const char *tmp_dir = getenv("TMP");
    const char *libretro_directory = getenv("LIBRETRO_DIRECTORY");
+   const char *libretro_assets_directory = getenv("LIBRETRO_ASSETS_DIRECTORY");
    if (!string_is_empty(tmp_dir))
       fill_pathname_expand_special(g_defaults.dirs[DEFAULT_DIR_CACHE],
          tmp_dir, sizeof(g_defaults.dirs[DEFAULT_DIR_CACHE]));
 
    gfx_set_dwm();
 
-   fill_pathname_expand_special(g_defaults.dirs[DEFAULT_DIR_ASSETS],
-      ":\\assets", sizeof(g_defaults.dirs[DEFAULT_DIR_ASSETS]));
+   if (!string_is_empty(libretro_assets_directory))
+      strlcpy(g_defaults.dirs[DEFAULT_DIR_ASSETS], libretro_assets_directory,
+	      sizeof(g_defaults.dirs[DEFAULT_DIR_ASSETS]));
+   else
+       fill_pathname_expand_special(
+	   g_defaults.dirs[DEFAULT_DIR_ASSETS],
+	   ":\\assets", sizeof(g_defaults.dirs[DEFAULT_DIR_ASSETS]));
    fill_pathname_expand_special(g_defaults.dirs[DEFAULT_DIR_AUDIO_FILTER],
       ":\\filters\\audio", sizeof(g_defaults.dirs[DEFAULT_DIR_AUDIO_FILTER]));
    fill_pathname_expand_special(g_defaults.dirs[DEFAULT_DIR_VIDEO_FILTER],