Repository: three.js
Issue: Renderer: Add MRT support to .readRenderTargetPixels().
Labels: Enhancement

<!-- Ignoring this template may result in your bug report getting deleted -->

**Describe the bug**
I used WebGLMultipleRenderTargets to output several textures , then I set one of these textures to WebGLRenderTarget's texture, 
then  I passed these WebGLRenderTexture to WebGLRenderer.readRenderTargetPixels to get one pixel’value, but I only got (0,0,0,0) from the Float32Array I have passed


**To Reproduce**

Steps to reproduce the behavior:
1、if "rt" is a WebGLRenderTarget, setRenderTarget to "rt",  eg. renderer.setRenderTarget(rt);
2、render something to it;
3、if I have another WebglRenderTarget "rt2", rt2.setTexture(rt.texture);
4、const read = new Float32Array( 4 );renderer.readRenderTargetPixels( rt2,posX, posY, 1, 1, new Float32Array( 4 ) );

or   you can copy following code to three.js/examples/webgl_read_float_buffer.html, and replace the script part. I have only changed a small of them

***Code***

```js
// code goes here
                       <script type="module">
			import * as THREE from '../build/three.module.js';

			import Stats from './jsm/libs/stats.module.js';

			let container, stats;

			let cameraRTT, sceneRTT, sceneScreen, renderer, zmesh1, zmesh2, rtTexture2;

			let mouseX = 0, mouseY = 0;

			const windowHalfX = window.innerWidth / 2;
			const windowHalfY = window.innerHeight / 2;

			let rtTexture, material, quad, materialScreen;

			let delta = 0.01;
			let valueNode;

			init();
			animate();

			function init() {

				container = document.getElementById( 'container' );

				cameraRTT = new THREE.OrthographicCamera( window.innerWidth / - 2, window.innerWidth / 2, window.innerHeight / 2, window.innerHeight / - 2, - 10000, 10000 );
				cameraRTT.position.z = 100;

				//

				sceneRTT = new THREE.Scene();
				sceneScreen = new THREE.Scene();

				let light = new THREE.DirectionalLight( 0xffffff );
				light.position.set( 0, 0, 1 ).normalize();
				sceneRTT.add( light );

				light = new THREE.DirectionalLight( 0xffaaaa, 1.5 );
				light.position.set( 0, 0, - 1 ).normalize();
				sceneRTT.add( light );

				rtTexture = new THREE.WebGLRenderTarget( window.innerWidth, window.innerHeight, { minFilter: THREE.LinearFilter, magFilter: THREE.NearestFilter, format: THREE.RGBAFormat, type: THREE.FloatType } );
				rtTexture2 = new THREE.WebGLRenderTarget( window.innerWidth, window.innerHeight, { minFilter: THREE.LinearFilter, magFilter: THREE.NearestFilter, format: THREE.RGBAFormat, type: THREE.FloatType } );

				material = new THREE.ShaderMaterial( {

					uniforms: { "time": { value: 0.0 } },
					vertexShader: document.getElementById( 'vertexShader' ).textContent,
					fragmentShader: document.getElementById( 'fragment_shader_pass_1' ).textContent

				} );

				materialScreen = new THREE.ShaderMaterial( {

					uniforms: { "tDiffuse": { value: rtTexture.texture } },
					vertexShader: document.getElementById( 'vertexShader' ).textContent,
					fragmentShader: document.getElementById( 'fragment_shader_screen' ).textContent,

					depthWrite: false

				} );

				const plane = new THREE.PlaneGeometry( window.innerWidth, window.innerHeight );

				quad = new THREE.Mesh( plane, material );
				quad.position.z = - 100;
				sceneRTT.add( quad );

				const geometry = new THREE.TorusGeometry( 100, 25, 15, 30 );

				const mat1 = new THREE.MeshPhongMaterial( { color: 0x555555, specular: 0xffaa00, shininess: 5 } );
				const mat2 = new THREE.MeshPhongMaterial( { color: 0x550000, specular: 0xff2200, shininess: 5 } );

				zmesh1 = new THREE.Mesh( geometry, mat1 );
				zmesh1.position.set( 0, 0, 100 );
				zmesh1.scale.set( 1.5, 1.5, 1.5 );
				sceneRTT.add( zmesh1 );

				zmesh2 = new THREE.Mesh( geometry, mat2 );
				zmesh2.position.set( 0, 150, 100 );
				zmesh2.scale.set( 0.75, 0.75, 0.75 );
				sceneRTT.add( zmesh2 );

				quad = new THREE.Mesh( plane, materialScreen );
				quad.position.z = - 100;
				sceneScreen.add( quad );

				renderer = new THREE.WebGLRenderer();
				renderer.setPixelRatio( window.devicePixelRatio );
				renderer.setSize( window.innerWidth, window.innerHeight );
				renderer.autoClear = false;

				container.appendChild( renderer.domElement );

				stats = new Stats();
				container.appendChild( stats.dom );

				valueNode = document.getElementById( 'values' );

				document.addEventListener( 'mousemove', onDocumentMouseMove );

			}

			function onDocumentMouseMove( event ) {

				mouseX = ( event.clientX - windowHalfX );
				mouseY = ( event.clientY - windowHalfY );
				// console.log(mouseX, mouseY);

			}

			//

			function animate() {

				requestAnimationFrame( animate );

				render();
				stats.update();

			}

			function render() {

				const time = Date.now() * 0.0015;

				if ( zmesh1 && zmesh2 ) {

					zmesh1.rotation.y = - time;
					zmesh2.rotation.y = - time + Math.PI / 2;

				}

				if ( material.uniforms[ "time" ].value > 1 || material.uniforms[ "time" ].value < 0 ) {

					delta *= - 1;

				}

				material.uniforms[ "time" ].value += delta;

				renderer.clear();

				// Render first scene into texture

				// renderer.setRenderTarget( rtTexture );
				renderer.setRenderTarget( rtTexture2 );
				renderer.clear();
				renderer.render( sceneRTT, cameraRTT );

				materialScreen.uniforms.tDiffuse.value = rtTexture2.texture;

				// Render full screen quad with generated texture

				renderer.setRenderTarget( null );
				renderer.render( sceneScreen, cameraRTT );

				const read = new Float32Array( 4 );
				console.log(windowHalfX + mouseX, windowHalfY - mouseY);
				rtTexture.setTexture(rtTexture2.texture);
				renderer.readRenderTargetPixels( rtTexture, windowHalfX + mouseX, windowHalfY - mouseY, 1, 1, read );

				valueNode.innerHTML = 'r:' + read[ 0 ] + '<br/>g:' + read[ 1 ] + '<br/>b:' + read[ 2 ];

			}
                       </script>
```

***Live example***
no live example,I cann't visit these links

* [jsfiddle-latest-release](https://jsfiddle.net/hyok6tvj/)
* [jsfiddle-dev](https://jsfiddle.net/c5m1kazu/)

**Expected behavior**
I hope I can get a effective value from the ArrayBufferView object, rather than 0.

**Screenshots**



**Platform:**

 - Device: [Desktop]
 - OS: [Windows10]
 - Browser: [Chrome]
 - Three.js version: [master, r130.1]