Fixed Tank Explosion Problem

Tank explosions did not show fully, this was due to the sprite not
resetting the rectangle of the texture it shows when a new texture was
loaded.

TODO:
Fix explosion sprite image
Fix horizontal/vertical line intersection problem
Fix corner collision problem
Delete rockets after they leave the screen
Delete rockets after they hit a tank
Make immovable crates to be used as boundaries
Make rockets explode if they hit crates head-on
Implement health system for crates and tanks
Make mines