Adds a new `Set Grease Pencil Color` node.
It was already possible to write to the color and opacity attributes using the
`Store Named Attribute` node, but this required the user to know the names
of the "built in" attributes.
Similar to how we can set other built-in attributes (e.g. the curve radius) this
adds a node to write to the following Grease Pencil attributes:
* `vertex_color` (point domain): The color of a point (the alpha value is used
as a mix factor with the base material color).
* `opacity` (point domain): The opacity of a point.
* `fill_color` (curve domain): The fill color of a stroke (the alpha value is used
as a mix factor with the base material color).
* `fill_opacity` (curve domain): The fill opacity of a stroke.
For consistency with other nodes, there is a mode to switch between writing
to the points (`Stroke` mode) and the fills (`Fill` mode).
Pull Request: https://projects.blender.org/blender/blender/pulls/136260
8a01c9b8ec