Handle differences between the glsl mod function and the metal::fmod function.

[expenses]

Utgh.

When using spirv-cross on a spir-v file that uses OpFMod, it emits the following metal function:

// Implementation of the GLSL mod() function, which is slightly different than Metal fmod()
template<typename Tx, typename Ty>
inline Tx mod(Tx x, Ty y)
{
    return x - y * floor(x / y);
}

Currently we emit metal::fmod for OpFMod functions. This results in some glitches in shaders. For example, the shadertoy shader https://www.shadertoy.com/view/XsXXDn looks like this with spirv-cross (using repi/shadertoy-browser#63):

But like this with naga:

[expenses]

From https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf table 6.4, it looks like metal uses x - y * trunc(x / y), and trunc has slightly different semantics. We should probably have a comment saying what the semantics of the IR is meant to be.

[kvark]

Oh I see, so this is about the sign. Filed gpuweb/gpuweb#1696
Let's change our SPIR-V frontend/backend to use OpFRem instead of OpFMod here.