atan(y, x) is not always returning expected results on NVIDIA OpenGL
drivers between versions 367 and 375. Work around this by emulating
atan(y, x) using the regular atan(x) function. A fix to the driver is
expected in a future release.

It is most convenient to implement the vector atan(y, x) functions by
using the scalar atan(y, x) function. Support for simple dependencies
between emulated functions is added to BuiltInFunctionEmulator. In the
current implementation one function is allowed to have at most one
other function as its dependency.

BUG=chromium:672380
TEST=angle_end2end_tests

Change-Id: I9eba8b0b7979c7c7eaed353b264932e41830beb1
Reviewed-on: https://chromium-review.googlesource.com/419016
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>