engine: handle swapping sequence files in LoadCacheFile for games with external StudioModelRenderer

ref/gl/gl_context.c

@@ -230,6 +230,8 @@ static int GL_RefGetParm( int parm, int arg )
 			return gl_texture_nearest.value == 0.0f;
 
 		return GL_TextureFilteringEnabled( R_GetTexture( arg ));
+	case PARM_GET_STUDIO_HDR:
+		return (intptr_t)R_StudioGetHeader();
 	default:
 		return ENGINE_GET_PARM_( parm, arg );
 	}

ref/gl/gl_local.h

@@ -403,6 +403,7 @@ void R_DrawSpriteModel( cl_entity_t *e );
 // gl_studio.c
 //
 void R_StudioInit( void );
+studiohdr_t *R_StudioGetHeader( void );
 void R_StudioLerpMovement( cl_entity_t *e, double time, vec3_t origin, vec3_t angles );
 struct mstudiotex_s *R_StudioGetTexture( cl_entity_t *e );
 int R_GetEntityRenderMode( cl_entity_t *ent );

ref/gl/gl_studio.c

@@ -2602,6 +2602,11 @@ static void R_StudioSetHeader( studiohdr_t *pheader )
 	m_fDoRemap = false;
 }
 
+studiohdr_t *R_StudioGetHeader( void )
+{
+	return m_pStudioHeader;
+}
+
 /*
 ===============
 R_StudioSetRenderModel