Fixed mouse not respecting slider's info.step.

[ActivexDiamond]

Sliders don't respect their info.step when dragged by the mouse. The first new line rounds value to the nearest step. The second new line also rounds fraction to the nearest step which is used for drawing. Skipping this line means the slider follows the mouse while being dragged but snaps back on release (the value is still always snapped).

The +0.5 is used to round it to the nearest value, removing it will always round down. value_changed is only updated if the value is actually changed; dragging the mouse less than info.step/2 will not cause an update.