title: WebGL FBO texture with transparent issue
body: <p>So the questions is :
  When I am using webgl to render stuff to a FBO, let's say a quad with 512 * 512, if I set the color to be (1, 0, 0, 0.5), which means its alpha is 0.5</p>
  <p>Then I use the color attachement as a texture for a sprite, then render it to screen, i found that the alpha does not work.</p>
  <p>I searched the question about premultiAlpha, but since my texture is not a image, i don't think it will work.</p>
  <p>Even if i use gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, true);</p>
  <p>Still not work.</p>
  <p>Any help?
  Thanks a lot</p>