Implement overridable scene render settings

This add a new set of (possible) render settings that can be defined at
the scene level and overridable at the scene layer level.

Once we get workspaces we can either add workspace inbetween scene and
scene layer evaluation. Or to replace layer settings, to avoid extra
confusion to users.

An example of this setting is "samples", as implemented now for the clay
engine.

release/scripts/startup/bl_ui/properties_render.py

@@ -584,6 +584,18 @@ class RENDER_PT_bake(RenderButtonsPanel, Panel):
             sub.prop(rd, "bake_user_scale", text="User Scale")
 
 
+class RENDER_PT_clay_layer_settings(RenderButtonsPanel, Panel):
+    bl_label = "Clay Layer Settings"
+    COMPAT_ENGINES = {'BLENDER_CLAY'}
+
+    def draw(self, context):
+        layout = self.layout
+        props = context.scene.layer_properties['BLENDER_CLAY']
+
+        col = layout.column()
+        col.prop(props, "ssao_samples")
+
+
 class RENDER_PT_clay_collection_settings(RenderButtonsPanel, Panel):
     bl_label = "Clay Collection Settings"
     COMPAT_ENGINES = {'BLENDER_CLAY'}
@@ -619,6 +631,7 @@ classes = (
     RENDER_PT_output,
     RENDER_PT_encoding,
     RENDER_PT_bake,
+    RENDER_PT_clay_layer_settings,
     RENDER_PT_clay_collection_settings,
 )