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

obs/window-basic-preview.cpp

@@ -59,6 +59,7 @@ static bool SceneItemHasVideo(obs_sceneitem_t *item)
 
 static bool CloseFloat(float a, float b, float epsilon=0.01)
 {
+	using std::abs;
 	return abs(a-b) <= epsilon;
 }