Blender Git Loki

GP Color Picker: Highlight the color under the mouse

Previously, the color picker provided no feedback on which
color under the mouse would get applied when clicking.
It also wasn't that clear what the current active color
was.

This commit fixes this, making the UI feel more like a
standard Blender menu/popup, and less like a glitchy
non-interactive graphic, by drawing a highlight rect
behind the item currently under the mouse. By default,
the current/active color will be highlighted before
the mouse enters the panel.