Add a checkered background to transparent artboards and the infinite canvas (#4022)

* Add checkered transparency rendering to infinite canvas and artboards

* Enable artboard clipping by default

* Make new infinite canvas documents begin with a white background layer

* Remove the export dialog's transparency option now that it's redundant

* Make exporting transparent JPGs use white not black

* Code review

editor/src/messages/tool/tool_messages/artboard_tool.rs

@@ -395,7 +395,7 @@ impl Fsm for ArtboardToolFsmState {
 							location: start.min(end).round().as_ivec2(),
 							dimensions: (start.round() - end.round()).abs().as_ivec2(),
 							background: graphene_std::Color::WHITE,
-							clip: false,
+							clip: true,
 						},
 					})
 				}