Unbreak CI: https submodule URLs, and three suites that assumed a display

CI has never gone green in the retained run history, and every failure is
the same 20-second death during checkout: six submodules -- SDL, SDL_image,
SDL_mixer, SDL_ttf, jansson, semver -- were registered with git@github.com:
SSH URLs, and the runner has no GitHub key. The https submodules (libccd,
tg, clay, and the two starfort ones) clone fine in the same run. All six now
use https, which is what every stanza added since already did; these are
public upstream repositories nothing here pushes to, so SSH bought nothing.

Second problem, waiting right behind the first: the character, sprite and
tilemap suites were the only three of seventeen that never set the dummy
video/audio/software-render hints, so on a headless runner they die in
SDL_Init with "No available video device" before testing anything. They now
set the same three hints the other fourteen suites set. (How this survived:
every local run happens on a machine with a display, and CI never got far
enough to run a test.)

Pre-flighted locally against every job the workflow defines, headless with
no driver variables in the environment, exactly as the runner would: Debug
with -Werror and coverage builds clean and all 37 tests pass including the
coverage fixtures; RelWithDebInfo with -Werror builds clean and the perf
suites pass their budgets at full scale; doxygen Doxyfile exits 0 under
FAIL_ON_WARNINGS; and the new ui suite runs under valgrind with zero errors
and no leaks.

Co-Authored-By: Claude Code (Claude Fable 5, claude-fable-5) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

tests/tilemap.c

@@ -740,6 +740,10 @@ int main(void)
 {
     PREPARE_ERROR(errctx);
 
+    SDL_SetHint(SDL_HINT_VIDEO_DRIVER, "dummy");
+    SDL_SetHint(SDL_HINT_AUDIO_DRIVER, "dummy");
+    SDL_SetHint(SDL_HINT_RENDER_DRIVER, "software");
+
     ATTEMPT {
 	CATCH(errctx, akgl_error_init());
 	akgl_gamemap = &akgl_default_gamemap;