Fixed a few bugs regarding UI when player insta-respawns.
Removed the addthis share dialog and also the share reward state.

For now, we're assuming that these dialogs won't prompt players to share
and they won't appreciate us putting the dialog in their face.

Also fixed a couple of bugs that were causing rendering flashes between
game states when the player first players through the tutorial.
Fixed bug where background isn't drawn when entering world map state.

Only show mode buttons when player has unlocked them.
Modified world map state to transition between difficulties smoothly.
Only draw lives and sparks in UI in challenge mode.
Show starting values when player hovers over world icon in hard mode.
Added half-star icons, using the same star icon as before with different
dimensions.