main/buffers: take into account FRONT_AND_BACK on ReadBuffer

main/buffers: take into account FRONT_AND_BACK on ReadBuffer

From OpenGL 3.1 spec, section 4.3.1 "Reading Pixels", page 190 (203 PDF)

  "When READ FRAMEBUFFER BINDING is zero, i.e. the default
   framebuffer, src must be one of the values listed in table 4.4,
   including NONE . FRONT_AND_BACK , FRONT , and LEFT refer to the
   front left buffer."

There is an equivalent text on OpenGL 4.5 spec, section 18.2.1
"Selecting Buffers for Reading", page 502 (524 PDF), so the behaviour
is still the same.

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

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