Used std::exchange for a thing

Note we don't really need to set the other object's renderer ptr to null since
the canvas dtor doesn't destroy it anymore, but still. Good to do.

src/gui/core/canvas.cpp

@@ -1375,15 +1375,13 @@ canvas::canvas(canvas&& c)
 	, w_(c.w_)
 	, h_(c.h_)
 	, texture_(std::move(c.texture_))
-	, renderer_(c.renderer_)
+	, renderer_(std::exchange(c.renderer_, nullptr))
 	, variables_(c.variables_)
 	, functions_(c.functions_)
 	, is_dirty_(c.is_dirty_)
 	, size_changed_(c.size_changed_)
 	, cache_invalidated_(c.cache_invalidated_)
 {
-	// Needed to ensure the other object doesn't destroy our software renderer prematurely.
-	c.renderer_ = nullptr;
 }
 
 canvas::~canvas()