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