Grease Pencil: Add Sample Color operator to Vertex Color Mode

This PR adds the `sample_color` operator to Grease Pencil Vertex Paint
Mode.

Pull Request: https://projects.blender.org/blender/blender/pulls/146951

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -4331,6 +4331,7 @@ def km_grease_pencil_vertex_paint(params):
          {"properties": [("data_path", "scene.tool_settings.use_gpencil_vertex_select_mask_segment")]}),
         # Flip primary and secondary color
         ("paint.brush_colors_flip", {"type": 'X', "value": 'PRESS'}, None),
+        ("paint.sample_color", {"type": 'X', "value": 'PRESS', "shift": True}, {"properties": [("merged", False)]}),
 
         # Edit Lines overlay
         ("wm.context_toggle", {"type": 'Q', "value": 'PRESS', "shift": True},

scripts/startup/bl_ui/space_view3d.py

@@ -2239,6 +2239,8 @@ class VIEW3D_MT_paint_vertex_grease_pencil(Menu):
         layout.operator("grease_pencil.vertex_color_levels", text="Levels")
         layout.operator("grease_pencil.vertex_color_hsv", text="Hue/Saturation/Value")
         layout.operator("grease_pencil.vertex_color_brightness_contrast", text="Brightness/Contrast")
+        layout.separator()
+        layout.operator("paint.sample_color").merged = False
 
 
 class VIEW3D_MT_select_paint_mask(Menu):