Per discussion on public_webgl mailing list, renamed EXT_draw_buffers to...

... WEBGL_draw_buffers and imposed the following implementation restrictions:

1) Minimum of 4 color attachments.
  2) MAX_COLOR_ATTACHMENTS <= MAX_DRAW_BUFFERS.
  3) Using RGBA/UNSIGNED_BYTE textures as color attachments must be supported.
  4) Using consecutive color attachments from COLOR_ATTACHMENT0 to COLOR_ATTACHMENT0+MAX_COLOR_ATTACHMENTS must be supported. (Holes are not guaranteed to be supported.)