[New Feature] Displaying briefing screen on mission start

[Rampastring]

Description:

Red Alert displays the briefing screen on mission start after the FMVs, which is handy. Tiberian Sun does not do this, which means that the player has to manually press Options -> Restate Briefing at the start of each mission, unless they realized the objectives from potential FMVs (which no mod has to a significant degree).

Tiberian Sun should also display the briefing screen on mission start.

Optional User/Mission Configuration:

This could potentially be made configurable through SUN.INI or even with a mission-based key ([Basic] ShowBriefingOnMissionStart= for example), although I personally would just leave it always enabled.

Additional Files:

Implemented in CnCNet ts-patches: https://github.com/CnCNet/ts-patches/blob/master/src/briefing_screen_mission_start.asm