According to both the GL 3.0 and ES 3.0 specifications (table 2.7 for GL
and table 2.8 for ES), the default value of BUFFER_ACCESS_FLAGS is
supposed to be zero.

Note that there are two related quantities: the obsolete BUFFER_ACCESS
enum and the new BUFFER_ACCESS_FLAGS bitfield.

BUFFER_ACCESS can only be GL_READ_ONLY, GL_WRITE_ONLY, or GL_READ_WRITE;
BUFFER_ACCESS_FLAGS can easily represent all three via GL_MAP_WRITE_BIT,
GL_MAP_READ_BIT, and their logical or.  It also supports more flags.

Thus, Mesa only stores the bitfield, and simply computes the old enum
when queried, via simplified_access_mode(bufObj->AccessFlags).

The tricky part is that, while BUFFER_ACCESS_FLAGS defaults to 0,
BUFFER_ACCESS defaults to GL_READ_WRITE for desktop [GL 3.0, table 2.8]
and GL_WRITE_ONLY_OES for ES [the GL_EXT_map_buffer_range extension].

Mesa tried to implement this by setting the default AccessFlags to
GL_MAP_READ_BIT | GL_MAP_WRITE_BIT on desktop, and GL_MAP_WRITE_BIT on
ES.  But in all specifications, it needs to be 0.

This patch moves that logic into simplified_access_mode(): when
AccessFlags == 0, it now returns GL_READ_WRITE for desktop and
GL_WRITE_ONLY for ES 1/2.  (BUFFER_ACCESS doesn't exist on ES 3.0,
so it's irrelevant there.)

With that in place, it changes the AccessFlags default to 0.

Fixes three es3conform tsets:
- copy_buffer_defaults
- map_buffer_range_modify_indices
- pixel_buffer_object_default_parameters

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