Fix some missing (transparent) text with libsdl-1.2.15_p20210224
e.g. empty menus in tuxmath
--- a/src/t4k_sdl.c
+++ b/src/t4k_sdl.c
@@ -1401,3 +1401,3 @@
   /* Use color key for eventual transparency: */
-  color_key = SDL_MapRGB(bg->format, 30, 30, 30);
+  color_key = SDL_MapRGBA(bg->format, 30, 30, 30, 0xff);
   SDL_FillRect(bg, NULL, color_key);
@@ -1448,3 +1448,3 @@
   SDL_SetColorKey(bg, SDL_SRCCOLORKEY|SDL_RLEACCEL, color_key);
-  out = SDL_DisplayFormatAlpha(bg);
+  out = SDL_DisplayFormat(bg);
   SDL_FreeSurface(bg);