spirv: Accept doubles in FaceForward, Reflect and Refract

spirv: Accept doubles in FaceForward, Reflect and Refract

The SPIR-V spec doesn’t specify a size requirement for these and the
equivalent functions in the GLSL spec have explicit alternatives for
doubles. Refract is a little bit more complicated due to the fact that
the final argument is always supposed to be a scalar 32- or 16- bit
float regardless of the other operands. However in practice it seems
there is a bug in glslang that makes it convert the argument to 64-bit
if you actually try to pass it a 32-bit value while the other
arguments are 64-bit. This adds an optional conversion of the final
argument in order to support any type.

These have been tested against the automatically generated tests of
glsl-4.00/execution/built-in-functions using the ARB_gl_spirv branch
which tests it with quite a large range of combinations.

The issue with glslang has been filed here:
https://github.com/KhronosGroup/glslang/issues/1279

v2: Convert the eta operand of Refract from any size in order to make
    it eventually cope with 16-bit floats.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>