xtures. Attach them as fbo's draw buffers.
    gl.bindFramebuffer(gl.DRAW_FRAMEBUFFER, fbo_draw);

    var drawbuffers = [gl.NONE, gl.NONE, gl.NONE, gl.NONE];
    if (drawbuffersFormatMask & srgbMask) {
        gl.bindTexture(gl.TEXTURE_2D, tex_srgb0);
        gl.texImage2D(gl.TEXTURE_2D, 0, gl.SRGB8_ALPHA8, size, size, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
        gl.framebufferTexture2D(gl.DRAW_FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tex_srgb0, 0);
        gl.bindTexture(gl.TEXTURE_2D, tex_srgb1)