SACT2: Do not put sprite -1 (main surface) in the scene list

Since this is the surface to render to, it should not be added to the
list of surfaces to draw from.

This is done by marking sprite -1 as hidden. Now SACT2.SP_GetShow(-1)
returns 0 and SACT2.SP_SetShow(-1, true) raises a runtime error. This is
consistent with the AliceSoft's implementation.

src/hll/SACT2.c

@@ -111,6 +111,7 @@ int sact_init(possibly_unused int cg_cache_size, bool chipmunk)
 	Texture *t = gfx_main_surface();
 	struct sact_sprite *sp = sact_create_sprite(0, t->w, t->h, 0, 0, 0, 255);
 	sp->texture = *t; // XXX: textures normally shouldn't be copied like this...
+	sprite_set_show(sp, false);
 
 	sprites++;
 
@@ -364,6 +365,8 @@ int sact_SP_SetBlendRate(int sp_no, int rate)
 
 int sact_SP_SetShow(int sp_no, int show)
 {
+	if (sp_no < 0)
+		VM_ERROR("Invalid sprite number: %d", sp_no);
 	struct sact_sprite *sp = sact_get_sprite(sp_no);
 	if (!sp) return 0;
 	sprite_set_show(sp, !!show);