vtn: Fix Modf.
vtn: Fix Modf.

We were botching this for negative numbers - floor of a negative rounds
the wrong way.  Additionally, both results are supposed to retain the
sign of the original.

To fix this, just take the abs of both values, then put the sign back.
There's probably a better way to do this, but this works for now.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler