v1.4 Performance optimisation

This release focuses on improving performance:

• All variables re-organised. Single letter names used for the most performance critical variables.
• Variables are defined in performance order for further speedups.
• Trajectory loop logic improvements. Technically some parts could be skipped more aggressively but I made a design choice that one projectile should not accelerate significantly when the other projectile lands.
• Trajectory drawing is now significantly faster - roughly double the previous speed - which has improved the feel of the game.
• Terrain generation speed improved.
• Sounds added for the shot and for the projectiles landing.
• Rendering of the castle hits has been switched to pre-created sprite strings of CHR$ control codes for each player. Hits are now drawn with very low latency, before colour clash from the shot can be seen.
• Castle sprites are now updated rather than being entirely redefined, and their colours are changed only as needed rather than for every screen re-draw.

1.4.1 minor update

• Fixed water bug in the terrain generation, and integrated the water extent detection into the main terrain generation loop
• Moved the terrain generation code further up the listing to improve performance

1.4.2 minor update

• Fixed magenta flicker as castles are overwritten during time of day change
• Faster time of day change when only brightness is changing

1.4.3 minor update

• Additional variable tidying, improved the CSV reference document to show context where vars are re-used
• Terrain traces edge to edge during generation before fill routine
• Sprite definitions moved to DATA statements to improve listing readability

1.4.4 minor update

• Fixed coordinate rounding regression in version 1.4.3