分辨率（2048x2048）
// 如果编程使用D3D渲染，请打开下面的设置，不然各个贴图接缝处有毛边。
// Clamp texture, don't wrap (otherwise edges can get filtered)
pd3dDevice->SetSamplerState(0, D3DSAMP_ADDRESSU, D3DTADDRESS_CLAMP);
pd3dDevice->SetSamplerState(0, D3DSAMP_ADDRESSV, D3DTADDRESS_CLAMP);
make skybox yourself:
Just set the FOV to 90, and rotate 90 deg in all directions and that's it.
FOV has to be 90 deg for the camera or it won't work right with a cube.
