普通に動画をLoadGraphして、グラフィックハンドルをテクスチャに指定すれば良いのではないでしょうか？
具体的には以下のような感じです。
//- 以下、サンプルコード ("clock.avi"を使用) -//
#include "DxLib.h"
const float CAM_MOVE_RATE = 5.0f;
int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow )
{
ChangeWindowMode( TRUE );
if ( DxLib_Init( ) == -1 ) return -1;
SetDrawScreen( DX_SCREEN_BACK );
int StrColor = GetColor( 255, 255, 255 );
VERTEX3D Vertex[4];
WORD Index[6] = { 0, 1, 2, 3, 2, 1 };
Vertex[0].pos = VGet( -200.0f, 200.0f, 0.0f );
Vertex[0].u = 0.0f;
Vertex[0].v = 0.0f;
Vertex[1].pos = VGet( 200.0f, 200.0f, 0.0f ) ;
Vertex[1].u = 1.0f;
Vertex[1].v = 0.0f;
Vertex[2].pos = VGet( -200.0f, -200.0f, 0.0f ) ;
Vertex[2].u = 0.0f;
Vertex[2].v = 1.0f;
Vertex[3].pos = VGet( 200.0f, -200.0f, 0.0f ) ;
Vertex[3].u = 1.0f;
Vertex[3].v = 1.0f;
Vertex[0].norm = Vertex[1].norm = Vertex[2].norm = Vertex[3].norm = VGet( 0.0f, 0.0f, -1.0f );
Vertex[0].dif = Vertex[1].dif = Vertex[2].dif = Vertex[3].dif = GetColorU8( 255, 255, 255, 255 );
Vertex[0].spc = Vertex[1].spc = Vertex[2].spc = Vertex[3].spc = GetColorU8( 0, 0, 0, 0 );
Vertex[0].su = Vertex[1].su = Vertex[2].su = Vertex[3].su = 0.0f;
Vertex[0].sv = Vertex[1].sv = Vertex[2].sv = Vertex[3].sv = 0.0f;
int mgh = LoadGraph( "clock.avi" );
PlayMovieToGraph( mgh );
int tgh = MakeScreen( 128, 128 );
float cx = 0.0f, cy = 0.0f;
while ( ProcessMessage( ) == 0 && CheckHitKey( KEY_INPUT_ESCAPE ) == 0 )
{
if ( !GetMovieStateToGraph( mgh ) )
break;
if ( CheckHitKey( KEY_INPUT_UP ) )
cy -= CAM_MOVE_RATE;
if ( CheckHitKey( KEY_INPUT_DOWN ) )
cy += CAM_MOVE_RATE;
if ( CheckHitKey( KEY_INPUT_LEFT ) )
cx -= CAM_MOVE_RATE;
if ( CheckHitKey( KEY_INPUT_RIGHT ) )
cx += CAM_MOVE_RATE;
SetDrawScreen( tgh );
ClearDrawScreen( );
DrawExtendGraph( 0, 0, 128, 128, mgh, FALSE );
SetDrawScreen( DX_SCREEN_BACK );
SetCameraPositionAndTargetAndUpVec( VGet( cx, cy, -800.0f ), VGet( 0.0f, 0.0f, 0.0f ), VGet( 0.0f, 1.0f, 0.0f ) );
ClearDrawScreen( );
DrawFormatString( 0, 0, StrColor, "test" );
DrawPolygonIndexed3D( Vertex, 4, Index, 2, tgh, FALSE );
ScreenFlip( );
}
DxLib_End( );
return 0;
}