This cl adds an implementation for:
- GLSLstd450InterpolateAtCentroid
- GLSLstd450InterpolateAtSample
- GLSLstd450InterpolateAtOffset

These functions essentially replicate the behavior of
regular interpolants in the fragment shader processing.

A specific extra difficulty encountered here is detecting
which kind of pointer offset we are dealing with. Pointer
offsets might be caused by [] operators being used on a
vector or on an array (possibly an array of vectors). This
distinction is important as it impacts what interpolant
offsets point to. Note that there's missing coverage in
dEQP-VK for interpolant arrays and this was caught with
SwANGLE tests (a dEQP-VK issue will be logged shortly).

Another issue was dealing with dynamic interpolant offsets,
which was solved by looping over all of them and combining
all plane equations into one before performing the
interpolation.

Bug: b/171415086
Change-Id: Id7c4c931918ba172d00da84655051445b110d3a9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51737
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>