glsl: add xfb_buffer compile time rules

glsl: add xfb_buffer compile time rules

Also copies the qualifier values to GLSL IR.

From the ARB_enhanced_layouts spec:

    "The *xfb_buffer* qualifier can be applied to the qualifier out,
    to output variables, to output blocks, and to output block
    members.  Shaders in the transform  feedback capturing mode have
    an initial global default of

        layout(xfb_buffer = 0) out;

    This default can be changed by declaring a different buffer with
    xfb_buffer on the interface qualifier out.  This is the only way
    the global default can be changed.  When a variable or output block
    is declared without an  xfb_buffer qualifier, it inherits the global
    default buffer.  When a variable or output block is declared with an
    xfb_buffer qualifier, it has that declared buffer.  All members of a
    block inherit the block's buffer.  A  member is allowed to declare
    an xfb_buffer, but it must match the buffer inherited from its
    block, or a compile-time error results.

    The *xfb_buffer* qualifier follows the same conventions, behavior,
    defaults, and inheritance rules as the qualifier stream, and the
    examples for stream apply here as well.  This includes a block's
    inheritance of the current global default buffer, a block member's
    inheritance of  the block's buffer, and the requirement that any
    *xfb_buffer* declared on a block member must match the buffer
    inherited from the block.

    ...

    It is a compile-time error to specify an *xfb_buffer* that is
    greater than  the implementation-dependent constant
    gl_MaxTransformFeedbackBuffers."

Reviewed-by: Dave Airlie <airlied@redhat.com>