Previously we would try to pass an entire struct through HLSL's
shader interface. Instead, split this off as if each field was
its own variable, which seems to be spec compliant (see ESSL 3.10).

In the future we may want to fix register packing to use specific
components of float4/int4/uint4 HLSL registers. This could also fix
the remaining bugs in the SM3 packing.

TEST=dEQP-GLES3.functional.shaders.linkage.varying.*
BUG=angleproject:910
BUG=angleproject:1202

Change-Id: I1fd8b4505abc39bd2385ed5c088c316d55d0bc2c
Reviewed-on: https://chromium-review.googlesource.com/311242Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>