Fix depthTexture types in WebGLRenderTarget.d.ts

[sam6321]

I noticed a few missing type definitions for render targets.
In this change:

• Added missing depthTexture argument to WebGLRenderTargetOptions.
• Changed depthTexture property on WebGLRenderTarget to be of DepthTexture type instead of Texture type.

I took a look inside of WebGLTextures and noticed the depthTexture property of a render target is a required to be a DepthTexture rather than just a Texture:

if ( ! ( renderTarget.depthTexture && renderTarget.depthTexture.isDepthTexture ) ) {

	throw new Error( 'renderTarget.depthTexture must be an instance of THREE.DepthTexture' );

}

[mrdoob]

Thanks!