Blender Git Loki

Node Editor: Style update to nodes

This patch changes how nodes look visually, in an attempt to fix a number of issues:
* The header background is currently drawn using a theme color fully opaque, this limits the colors we can use because the node name/label is drawn on top.
* Hard-coded transparency makes nodes hard to read. The node backdrop already has alpha so if the user wants it they can set it. This patch uses alpha from the theme.
* Better muted status indicator, instead of simply making everything transparent and the wires inside red, draw a red outline around the node, darken the header and backdrop.
* On muted nodes, display wires behind the backdrop to not interfere with text/widgets inside the node.

Nodes:

* Darken header to improve readability of node label.
* Draw a line under the header
* Thicker outline.
* Do not hard-code transparency on nodes, use the theme's node backdrop alpha component.
* Use angle icon instead of triangle (to be consistent with the [[ https://developer.blender.org/D12814 | changes ]] to panels)

Style adjustment to sockets drawing:

* Do not hard-code the socket outline color to black, use `TH_WIRE` instead
* Do not use `TH_TEXT_HI` for selected sockets, use `TH_ACTIVE` (active node outline)
* Do not draw sockets background transparent on muted nodes.
* Thicker outline to help contrast and readability

{F11496707, size=full}

Reviewed By: #user_interface, HooglyBoogly

Differential Revision: https://developer.blender.org/D12884