Change the top coordinate of the animation channel list UI elements to
`rect->ymin` so they correctly span from `rect->ymin` to
`channel_height`.
Some buttons in the dope sheet channels didn't scale properly with the
`Keyframe Scale Factor` preferences setting. This was caused by using
the `ymid` value (`ymid = BLI_rctf_cent_y(rect) - 0.5f * ICON_WIDTH`) to
position the buttons that supposed to fill all vertical space in the
channel (with `channel_height` height). The `ymid` value is only
appropriate for the UI elements that with `ICON_WIDTH` height.
Maniphest Tasks: T83716
Reviewed by: sybren
Differential Revision: https://developer.blender.org/D9841
95afc53604