Implemented saving/loading from checkpoints.
- A checkpoint (track position) can be set in the options screen, and also while playing by pressing "CTRL+S".
- A checkpoint can be loaded by pressing "CTRL+L" while playing.  This will reset all game data and begin from the checkpoint time; the ranking screen will be skipped upon song completion.

Other changes:
- Don't draw grade if no objects have been processed (previously defaulted to GRADE_D).
- Calculate slider start/end angles based on a step difference (previously an arbitrary 0.01 difference).
- Always end a slider curve base on the last control point.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>