glsl: add xfb_stride compile time rules

glsl: add xfb_stride compile time rules

From the ARB_enhanced_layouts spec:

   "The *xfb_stride* qualifier specifies how many bytes are consumed
   by each captured vertex.  It applies to the transform feedback
   buffer for that declaration, whether it is inherited or explicitly
   declared. It can be applied to variables, blocks, block members,
   or just the qualifier out.  If the buffer is capturing any
   double-typed outputs, the stride must be a multiple of 8, otherwise
   it must be a multiple of 4, or a compile-time or link-time error
   results.

   ...

   The resulting stride (implicit or explicit) must be less than or
   equal to the implementation-dependent constant
   gl_MaxTransformFeedbackInterleavedComponents."

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