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