glsl: add xfb_stride link time validation

glsl: add xfb_stride link time validation

From the ARB_enhanced_layous spec:

   "It is a compile-time or link-time error to have any *xfb_offset*
    that overflows *xfb_stride*, whether stated on declarations before
    or after the *xfb_stride*, or in different compilation units.

    ...

    When no *xfb_stride* is specified for a buffer, the stride of a
    buffer will be the smallest needed to hold the variable placed at
    the highest offset, including any required padding."

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