GPencil: Implement Holdout materials

This new feature allows to use the strokes as an eraser of any stroke below. This is very handy to open holes in filled areas.

After running some tests, we have decided to keep the additive effect of the holdout color. To get clean holdout areas, just move the color to black to remove any additive effect. To have additive effect can be used in situations like tint slightly a transparent window with blue to simulate the glass.

See T79878 for more details

Differential Revision: https://developer.blender.org/D8932

release/scripts/startup/bl_ui/properties_material_gpencil.py

@@ -70,6 +70,7 @@ class GPENCIL_UL_matslots(UIList):
                 row.prop(ma, "name", text="", emboss=False, icon_value=icon)
 
                 row = layout.row(align=True)
+
                 if gpcolor.ghost is True:
                     icon = 'ONIONSKIN_OFF'
                 else:
@@ -168,6 +169,8 @@ class MATERIAL_PT_gpencil_strokecolor(GPMaterialButtonsPanel, Panel):
             if gpcolor.mode == 'LINE':
                 col.prop(gpcolor, "use_overlap_strokes")
 
+            col.prop(gpcolor, "use_stroke_holdout")
+
 
 class MATERIAL_PT_gpencil_fillcolor(GPMaterialButtonsPanel, Panel):
     bl_label = "Fill"
@@ -222,6 +225,8 @@ class MATERIAL_PT_gpencil_fillcolor(GPMaterialButtonsPanel, Panel):
             col.prop(gpcolor, "texture_scale", text="Scale")
             col.prop(gpcolor, "texture_clamp", text="Clip Image")
 
+        col.prop(gpcolor, "use_fill_holdout")
+
 
 class MATERIAL_PT_gpencil_preview(GPMaterialButtonsPanel, Panel):
     bl_label = "Preview"