enhancement: Allow playing billboard animation on ground textures

[mefistotelis]

In pre-alpha version of the game, animation plays on a ground structure. The ground uses texture with Slot=5 which contains the animation. However, playback isn't in working in SW Port because the ground playback does not set ingame.Flags |= GamF_Unkn00040000;. Without this flag set, the game assumes that all billboards are off-screen and skips decoding a next frame of animation.

When ground texture using 'Slot=5' is drawn on screen, it should set the flag GamF_Unkn00040000.

Low priority, as this is only used in pre-alpha level.

While at it, the GamF_Unkn00040000 setting for objects should be improved as well - currently animation can only play on an object with type BILLBOARD rather than any object which just happens to use the proper texture index.
That is also visible in pre-alpha, where the billboard remains black due to wrong type.