fix memory leak problem with score display

when displaying a new score the old texture was not freed
use of accessors for game items dimensions

new functions for getting items (ball, brick and player) have been added:
player_heigth, player width, ball_radius, brick_width, brick_heigth

also functions to get the center position of player and ball:
player_center_x, ball_center_x and ball_center_y

function usings these values are now more easy to understand