Make default skin cursor more visible against bright backgrounds

[peppy]

• Adds a dark colour to the centre (similar to stable) to make the default cursor more usable.
• Fixes cursor scale potentially not being read (only happens in tests).
• Fixes cursor tests not correctly showing transforms due to calling OnPressed/OnReleased too often.
• Adds tests against random background colours (and user input driven tests).
• Depends on Add the ability to disable the ManualInputManager cursor guide osu-framework#3706

This is not a final design, just a temporary usability improvement until we have a more definitive cursor design.

[smoogipoo]

lgtm, can read much better with this