Fixed diameter-agnostic ball collisions. Replaced structs with global variables

-Code now makes use of constant global variables to reduce the necessary arithmetic operations, and other global variables instead of structs and struct pointers.
-The ball position is now specified by its top left corner instead of its centre, allowing for even diameters if desired. The game should now operate correctly with any reasonable diameter chosen.
-In this commit, the print_test function is not called, usleep is called, and the reset_ball function includes a print statement to indicate that it has been called. The first two are consistent with what the final product will do. However, all three changes were made for the purpose of testing the program's processing load on the raspberry pi, and should be reverted for normal testing and modifications.