message: Normalizes drawBuffers for WebGL 1 & 2
diff: import System from '../System';
  import { Rectangle } from '@pixi/math';
  +import { ENV } from '@pixi/constants';
  +import { settings } from '../settings';
  /**
  * Framebuffer system
  @@ -16,11 +18,38 @@ export default class FramebufferSystem extends System
  */
  contextChange()
  {
  - this.gl = this.renderer.gl;
  + const gl = this.gl = this.renderer.gl;
  +
  this.CONTEXT_UID = this.renderer.CONTEXT_UID;
  this.current = null;
  this.viewport = new Rectangle();
  - this.drawBufferExtension = this.renderer.context.extensions.drawBuffers;
  + this.hasMRT = true;
  +
  + // webgl2
  + if (!gl.drawBuffers)
  + {
  + // webgl 1!
  + let nativeDrawBuffersExtension = this.renderer.context.extensions.drawBuffers;
  +
  + if (settings.PREFER_ENV === ENV.WEBGL_LEGACY)
  + {
  + nativeDrawBuffersExtension = null;
  + }
  +
  + if (nativeDrawBuffersExtension)
  + {
  + gl.drawBuffers = (activeTextures) =>
  + nativeDrawBuffersExtension.drawBuffersWEBGL(activeTextures);
  + }
  + else
  + {
  + this.hasMRT = false;
  + gl.drawBuffers = () =>
  + {
  + // empty
  + };
  + }
  + }
  }
  /**
  @@ -252,9 +281,9 @@ export default class FramebufferSystem extends System
  activeTextures.push(gl.COLOR_ATTACHMENT0 + i);
  }
  - if (this.drawBufferExtension && activeTextures.length > 1)
  + if (activeTextures.length > 1)
  {
  - this.drawBufferExtension.drawBuffersWEBGL(activeTextures);
  + gl.drawBuffers(activeTextures);
  }
  if (framebuffer.depthTexture)
  