windows: fix Unicode function and type inconsistencies

The surrounding code in all of these instances expects the Unicode
variants. Previously, this code mixed Unicode and ANSI/ASCII calls if
`UNICODE` was undefined, which caused type and logic errors. Explicitly
spelling out the W removes any reliance on that macro.

src/camera/mediafoundation/SDL_camera_mediafoundation.c

@@ -741,7 +741,7 @@ static bool MEDIAFOUNDATION_OpenDevice(SDL_Camera *device, const SDL_CameraSpec
     SDL_Log("CAMERA: opening device with symlink of '%s'", utf8symlink);
     #endif
 
-    wstrsymlink = WIN_UTF8ToString(utf8symlink);
+    wstrsymlink = WIN_UTF8ToStringW(utf8symlink);
     if (!wstrsymlink) {
         goto failed;
     }
@@ -901,7 +901,7 @@ static char *QueryActivationObjectString(IMFActivate *activation, const GUID *pg
         return NULL;
     }
 
-    char *utf8str = WIN_StringToUTF8(wstr);
+    char *utf8str = WIN_StringToUTF8W(wstr);
     CoTaskMemFree(wstr);
     return utf8str;
 }