glsl: buffer variables can be readonly and writeonly

glsl: buffer variables can be readonly and writeonly

In GLSL ES 3.10 session 4.9 [Memory Access Qualifiers], it has the
following description:

"A variable could be qualified as both readonly and writeonly,
disallowing both read and write, but still be passed to
imageSize() to have the size queried.".

This is for image variable, but not for buffer variables.

According to https://github.com/KhronosGroup/OpenGL-API/issues/7 Khronos
intent is to allow both readonly and writeonly in buffer variables, and
as such it will update the GLSL specification.

This commit address this issue, and fixes:

KHR-GL{43,44,45}.shader_storage_buffer_object.basic-readonly-writeonly
KHR-GLES31.core.shader_storage_buffer_object.basic-readonly-writeonly

v2: set correctly fields[i] memory flags (Samuel Pitoiset).

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>