Made sure varyings had the same interpolation qualifier in fragment
shader and vertex shader (either both flat or both smooth ).

Fixes:
dEQP-GLES3.functional.shaders.linkage.varying.rules.interpolation_mismatch_1

Change-Id: I7f68490dc19a3365e492b666acda6f5db91d10ab
Reviewed-on: https://swiftshader-review.googlesource.com/15768Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>