Render 3D and texture arrays with MRT

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Originally did a PR for it but turns out that we can render 3D and texture arrays with MRT without any modifications to the WebGLRenderer. I made a demo to showcase

Demo - https://wizgrav.github.io/three-mrt/

Code snippet:

function renderTo3DTexture() {

    // this sets layer 0 to COLOR_ATTACHMENT0 
    renderer.setRenderTarget( renderTarget );
    
    const gl = renderer.getContext();
    const drawBuffersArray = [ gl.COLOR_ATTACHMENT0 ];
    const glTexture = renderer.properties.get( renderTarget.texture ).__webglTexture;
    
    // set the rest of the layers to the corresponding attachments
    for ( let i = 1; i < 4; i ++ ) {
    
	    gl.framebufferTextureLayer( gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0 + i, glTexture, 0, i );
    
	    drawBuffersArray.push( gl.COLOR_ATTACHMENT0 + i );
    
    }
    
    // set draw buffers state for MRT
    gl.drawBuffers( drawBuffersArray );
    
    renderer.render( postProcessScene, postProcessCamera );
    
    // restore draw buffers state
    gl.drawBuffers( [ gl.COLOR_ATTACHMENT0 ] );
    
    renderer.setRenderTarget( null );
  
  }