Varyings and in/out variables that are passed between shader pipeline
stages are verified to have matching types at link-time, even when only
statically referenced (this includes trivially optimized out branches)
or even just declared.

GLSL ES 3.00 - 4.3.10 Linking of Vertex Outputs and Fragment Inputs

Change-Id: I122b1cdcc4630c86a8ebfb4d4e37f3a7a335afbe
Reviewed-on: https://swiftshader-review.googlesource.com/16070Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>