Fix attract title logo slidein (resolves #1273)

- V_SLIDEIN: For (G_IsTitleCardAvailable == false)/(versus at map start == true) contexts, use timeinmap instead of leveltime
- Don't reset timeinmap when reloading gamestate, so it's more functionally equivalent to lt_ticker
These two changes also fix the hypothetical issue of titlecard-less custom gametypes having weird HUD state on late netgame join.

Timeinmap is exclusively used for HUD, as is st_fadein. timeinmap also doesn't increment under g_fastforward, therefore the best candidate to tie together