`ui_draw_but_CURVE` used a flat color shader to draw points. This
isn't valid as that shader doesn't support point rendering.
This is fixed to replace the `GPU_SHADER_3D_FLAT_COLOR` with
`GPU_SHADER_3D_POINT_VARYING_SIZE_VARYING_COLOR`.
Regression found when loading `monster.blend` using Vulkan. This
scene has the color management tab open with a custom curve.
Pull Request: https://projects.blender.org/blender/blender/pulls/124025
459b4c7f00