UI: Fix precision in float comparison helper

Use std::abs instead of abs to avoid loss in precision and also fix
the corresponding warning from clang (3.5.0):

warning: using integer absolute value function 'abs' when argument is
of floating point type [-Wabsolute-value]

Closes jp9000/obs-studio#414
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f