Add Mouse Support to FixedRoom2D

[MarkOates]

wip

[MarkOates]

Closing. The feature was added with the following commits:

• 2823c4f - Translate mouse movement to cursor movement
• c5d325c - Add clamp function to mouse cursor, add room size dimensions
• 8172a73 - clamp the mouse cursor when moved (at the Screen level)

Now, note this is not an ideal implementation (which is part of the reason it took longer than expected). The design flaws are listed in this issue and are expected to be addressed before the feature is actually considered "finished".