Added TIME as global shader param, renamed '_physics_update' to '_fixed_update', and bug fixes.

[Chukobyte]

• Added TIME as global shader param to screen and sprite shader types.
• Renamed _physics_update to _fixed_update to more accurately describe fixed time step.
• Added set_current_animation_frame function to AnimatedSprite nodes.
• Bug fixes:
  • Fixed bug with calling play on AnimatedSprite didn't actually play the animation when it was stopped.
  • Fixed bug with crashing stop function on AnimatedSprite.
  • Fixed issue with setting up initial shader params.