Added option to always update the mouse picking via saving the last m…

[chaoticgood1]

Hi! We implemented the ff:

• Added option to always update the mouse picking even when mouse is not moved via saving the last mouse position
• By default it is false.
• Snippet to set as always_on:
  .with(PickSource { always_on: true, ..Default::default() })
• Solves issues: Mouse picking is not updated when mouse is not moved and object is deleted #64 and Mouse picking intersection doesn't update on camera move #67

[aevyrie]

Thanks! What is the impact of defaulting to always on?

For example, if the window is not active, will it continue to check for intersections every frame? I think the answer is yes.

I ask because based on @Sheepyhead's use case, they would need it to be enabled, and it might actually make sense if this is default behavior. However, my only worry is that it will always be running even when the application isn't being used. This isn't an issue now, because mouse events are not sent when the window is inactive.

[aevyrie]

Looks like the multiple_windows.rs example needs a small fix with this change.
https://github.com/aevyrie/bevy_mod_picking/pull/68/checks?check_run_id=1540877088#step:6:405

199 |             PickMethod::CameraCursor(WindowId::primary()),
    |             ^^^^^^^^^^^^^^^^^^^^^^^^ ------------------- supplied 1 argument
    |             |
    |             expected 2 arguments

217 |             PickMethod::CameraCursor(window_id),
    |             ^^^^^^^^^^^^^^^^^^^^^^^^ --------- supplied 1 argument
    |             |
    |             expected 2 arguments

run cargo test on your branch to see the errors.