Fix up test scrolling screen

Move the camera boundaries to fit the background

Extend the ground surface

Add a ceiling (because the level is inside a house) using horizontal
borders
Extract border with of 0.06f into a constant BORDER_WIDTH = 0.06f
Add walls to test scrolling screen

Walls are used instead of setting up borders because vertical borders in
a horizontally scrolling screen would push all of the entities as the
player moves across the screen