Clear last-mouse-moved pressed button when that button gets a mouse-up

This fixes an annoying issue where if the last mouse moved event was
during a drag it would never trigger mouse cursor changes until next
mouse move reset it. It makes sense to continue to not change the cursor
while the button is pressed so instead this tracks when the mouse button
is released in order to update the mouse move event

crates/gpui/src/platform/event.rs

@@ -64,7 +64,7 @@ impl Default for MouseButton {
     }
 }
 
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Copy, Debug, Default)]
 pub struct MouseButtonEvent {
     pub button: MouseButton,
     pub position: Vector2F,