main/buffers: update error handling on DrawBuffers for 4.5

main/buffers: update error handling on DrawBuffers for 4.5

Before 4.5, GL_BACK was not allowed as a value of bufs. Since 4.5 it
is allowed under some circumstances:

From the OpenGL 4.5 specification, Section 17.4.1 "Selecting Buffers
for Writing", page 493 (page 515 of the PDF):
 "An INVALID_ENUM error is generated if any value in bufs is FRONT,
  LEFT, RIGHT, or FRONT_AND_BACK . This restriction applies to both
  the de- fault framebuffer and framebuffer objects, and exists
  because these constants may themselves refer to multiple buffers, as
  shown in table 17.4."

And on page 492 (page 514 of the PDF):
 "If the default framebuffer is affected, then each of the constants
  must be one of the values listed in table 17.6 or the special value
  BACK . When BACK is used, n must be 1 and color values are written
  into the left buffer for single-buffered contexts, or into the back
  left buffer for double-buffered contexts."

This patch keeps the same behaviour if OpenGL version is < 4. We
assume that for 4.x this is the intended behaviour, so a fix, but for
3.x the intended behaviour is the already in place.

Part of the fix for:
GL45-CTS.direct_state_access.framebuffers_draw_read_buffers_errors

v2: remove forgot printf
v3: remove spaces before commas on spec quote, split line too
    long (Anuj)

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>