Caused by 364d62e59b.
In draw mode, we want to make sure to show vertex colors by default
because users can draw with them. Otherwise this can lead to unexpected
behavior in solid shading mode when vertex colors used to not be
rendered in draw mode.
The previous fix tried to do this by enforcing the `V3D_SHADING_VERTEX_COLOR`
mode in draw mode. But this has the side effect that the material
stroke and fill color are overriden with pure white.
To fix the issue correctly, simply ensure that `vert_col_opacity` in
`grease_pencil_layer_cache_add` is set to 1.0f. This means that
vertex colors and material colors are shown just like in the render.
Also resolves#128680.
Pull Request: https://projects.blender.org/blender/blender/pulls/129054
715a37b662