Fix downward dash not damaging enemies and add post-dash invincibility

Stomping was guarded by the invincibility check, so during a downward
dash the player could never deal stomp damage. Move the invincibility
guard to only protect against taking damage, not dealing it.

Extend dash invincibility by PLAYER_DASH_INV_GRACE (0.15s) past the
dash duration so the player is briefly protected after landing.

Closes #15

src/game/player.h

@@ -39,6 +39,7 @@
 
 /* Invincibility after taking damage */
 #define PLAYER_INV_TIME 1.5f       /* seconds of invincibility      */
+#define PLAYER_DASH_INV_GRACE 0.15f /* extra invincibility after dash */
 
 /* Aim direction (for shooting) */
 typedef enum AimDir {