glsl: add new interpolateAt* builtin functions

V2: - Don't assume everyone wants interpolateAtSample() lowered to
      interpolateAtOffset. It turns out this isn't what we want most
      of the time for i965. Lowering can be added later in an ir pass
      which drivers opt into, rather than bolting it straight into the
      builtin definition.
    - Only expose the interpolateAt* builtins in the fragment language.

Signed-off-by: Chris Forbes <[email protected]>
Reviewed-by: Ilia Mirkin <[email protected]>

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