update camera projection if viewport changed

[jakobhellermann]

fixes https://github.com/bevyengine/bevy/issues/5944

Uses the second solution:

2. keep track of the old viewport in the computed_state, and if camera.viewport != camera.computed_state.old_viewport, then update the projection. This is more reliable, but needs to store two UVec2s more in the camera (probably not a big deal).