UISliderTemplate displays no borders due to missing atlases

[Meorawr]

UISliderTemplate makes use of the SliderBar nineslice atlas texture kit which is either entirely missing or incomplete in the Classic data. As a result, attempting to display any slider that inherits this template results in disappointment and missing border textures as shown below.

In 1.15.4.56419 the following atlases are missing, resulting in the above screenshot:

• SliderBar-NineSlice-CornerBottomLeft
• SliderBar-NineSlice-CornerBottomRight
• SliderBar-NineSlice-CornerTopLeft
• SliderBar-NineSlice-CornerTopRight
• _SliderBar-NineSlice-EdgeBottom
• _SliderBar-NineSlice-EdgeTop
• !SliderBar-NineSlice-EdgeLeft
• !SliderBar-NineSlice-EdgeRight

In 4.4.0 only the tiled vertical edge atlases are missing. For consistency with Mainline, it'd be good to have these added in case someone is using vertical sliders.

• !SliderBar-NineSlice-EdgeLeft
• !SliderBar-NineSlice-EdgeRight

[Meorawr]

Partially resolved in 1.15.4.56493 with the addition of the missing atlases used by UISliderTemplate itself, however the vertically tiled parts are still missing.

Vertical sliders feel rare enough to not warrant keeping this open longer than necessary.