mesa: Rework crazy error code rules in glDrawBuffers().

mesa: Rework crazy error code rules in glDrawBuffers().

Perhaps most importantly, this patch adds comments quoting the relevant
spec paragraphs above each error condition.

It also makes three changes:
- For FBOs, GL_COLOR_ATTACHMENTm where m >= MaxDrawBuffers is supposed
  to generate INVALID_OPERATION (not INVALID_ENUM).
- Constants that refer to multiple buffers (such as FRONT, BACK, LEFT,
  RIGHT, and FRONT_AND_BACK) are supposed to generate INVALID_OPERATION,
  not INVALID_ENUM.
- In ES 3.0, for FBOs, buffers[i] must be NONE or GL_COLOR_ATTACHMENTi
  or else INVALID_OPERATION occurs.  (This is a new restriction.)

Fixes es3conform's draw-buffers-api test.

v2: The error path was missing a "return" like all the other error
paths.  Also, we may as well call it glDrawBuffers in the error message
since the ARB suffix doesn't exist in ES 3.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>