689fce50bd
Since image vectorscope RGB mode addition (567455124d), changing
the opacity was causing the scope to get recalculated from scratch,
because opacity value was put into vecscope_rgb data directly.
Instead of that, make vecscope_rgb data only contain RGB, and fill in
the GPU vertex buffer alpha values when creating the GPU batch.
Now tweaking the scope opacity slider feels at about the same
performance as in 4.0.
Pull Request: https://projects.blender.org/blender/blender/pulls/120854