glsl: Allow varying structs in GLSL ES 3.00 and GLSL 1.50.

Previously I thought that varying structs had been added to GLSL ES
3.00 by mistake, because chapter 11 of the GLSL ES 3.00 spec
("Counting of Inputs and Outputs") failed to mention how structs
should be handled.  Khronos has clarified
(https://cvs.khronos.org/bugzilla/show_bug.cgi?id=9828) that varying
structs are indeed required, and that chapter 11 will be modified to
indicate that the minimal reference packing algorithm flattens varying
structs to their individual components.

Mesa doesn't flatten varying structs to their individual components,
but this is ok, since it packs varyings of all kinds with no wasted
space at all (except where this is impossible due to differing
interpolation modes), so it will outperform the minimal reference
packing algorithm in all but the most pathological cases.