st/scroll-view-fade: Fix vertical top fading

The fade for the vertical top edge was calculating the fade ratio for a larger height (up to where the bottom fade starts) than it was displaying.

This is a regression introduced in 5d65a424. There are some simplifications that could be done to the shader: removing vfade_scale and hfade_scale and changing (fade_area_topleft[1] - y) / (fade_area_topleft[1] - fade_top_start) to (y - fade_area_topleft[1]) / vfade_offset etc. However the former is already done in !1630 (merged) and the latter would just cause an unnecessary rebase for that MR. Also the compiler should be able to optimize that anyway.