Added PVRTC extension conformance test
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.)
Merge pull request #219 from kenrussell/drawbuffers

Per discussion on public_webgl mailing list, renamed EXT_draw_buffers to...
Forgot to add test to test list
Merge pull request #218 from toji/pvrtc

Added PVRTC extension conformance test