glsl: apply xfb_stride to implicit offsets for ifc block members

glsl: apply xfb_stride to implicit offsets for ifc block members

When we have an interface block like:

layout (xfb_buffer = 0, xfb_offset = 0) out Block {
                             vec4 var1;
    layout (xfb_stride = 32) vec4 var2;
                             vec4 var3;
};

We take into account the stride of var2 when calculating the offset
for var3.

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