Duke3D: Fix flicker in automatic view angle

This MR fixes two small issues when an automatic view angle is calculated, typically when going up or down a slope.

• Sometimes the game calculates an excessive slope difference between the current view angle and the slope angle, leading to a vertical flicker in the image momentarily. This is specially noticeable in E1L6, for example, when going up the slope to the force field control tower, which uses horizontal sprites to cover gaps.

• To make movement smoother, apply a small delay (100ms) when starting to adjust the view angle automatically.

Fixes #265 (closed)

Video showcasing the issue: https://rg3.name/pub/duke3d-slope-flicker/slope-flicker.mkv

After applying this fix: https://rg3.name/pub/duke3d-slope-flicker/slope-flicker-fixed.mkv