deleteFramebuffer shouldn't throw if GL context is lost/destroyed

[gre]

If I destroy the GL context earlier, a fbo.dispose() throw an error, which is not the case in WebGL.

   Invalid GL context
        at Error (native)

      at WebGLFramebuffer.Object.<anonymous>.WebGLFramebuffer._performDelete (node_modules/gl/webgl.js:1743:22)
      at checkDelete (node_modules/gl/webgl.js:505:9)
      at WebGLRenderingContext.deleteFramebuffer (node_modules/gl/webgl.js:1762:3)
      at Framebuffer.Object.<anonymous>.proto.dispose (node_modules/gl-fbo/fbo.js:357:6)