constexpr NaN,+Inf,-Inf

Reading extern values meant these couldn't be compile-time constants.

math.h has INFINITY, which is macro that is supposed to expand to float +inf.
On MSVC it seems it's natively a double, so we cast just to make sure.

There's nan(const char*) in math.h for NaN too, but I don't trust that
to be compile-time evaluated.  So instead, we keep reinterpreting a bit pattern.

I did try to write

    static constexpr float float_nan() { ... }

and completely failed.  constexpr seems a bit too restrictive in C++11 to make
it work, but Clang kept telling me, you'll be able to do this with C++14.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2233853002

Review-Url: https://codereview.chromium.org/2233853002

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