This update originally was meant to just improve the startup code, but testing the game inspired new ideas and also uncovered some flaws never seen before, so it ended up being quite substantial and brings SkillGrid to yet another level!

v1.4 (6.1.2024)
1. Added cells field flashing when these cells (are about to) appear:
* ASTEROIDS cell (bright colors);
* BEAM/* cells, if their exit from the bottom of the screen would reduce the number of beams shot by the spaceship (bright colors);
* MOTHERSHIP, MUSIC MODE, SKULL (same colors of the cells field when the associated boss/mode is active).
2. Made the spaceship flash red when its shield is low.
3. Made the spaceship glow red for half a second when its shield gets damaged.
4. Made the middle layer appear progressively at the beginning of a game.
5. Made the cells layer populate with empty cells progressively.
6. Changed a few pixels of the title screen spaceship icon and of the Skull.
7. Improved the handling of audio channels.
8. Made sure no sprite tearing occurs when switching between screens.
9. Made sure that the system MMU setup does not interfere.
10. Fixed crash when a tune was started while another tune was playing (e.g. when the MUSIC MODE cell was caught while missiles were active).
11. Fixed the stopping of the music mode music (it inhibited the restart of the spaceship afterburner sound).
12. Fixed the X position of specifically-requested cells (it was equal to the type of the cell removed at the same time).
13. Fixed the lengths of some sounds samples (they were odd).
14. Made various speed and memory optimizations.
15. Made the startup and cleanup code more robust.
16. Worked on the cells:
* improved the handling of the TURN cell (now it is randomized and it ensures that the speeds of the UFOs will always change);
* increased the frequency of the MUSIC MODE cells:
* decoupled the randomization of the cells types and positions;
* created the BOSSES (virtual) class of cells, grouping the BOLTSPITTER and SKULL cells (previously MALUSES) and the MOTHERSHIP cell (previously UNDEFINED) into it;
* made the template of the BOLTSPITTER and SKULL cells striped;
* made the template of the TURN cell fringed:
* made the template of the BEAM/DOUBLE cell equal to that of the other BEAM/* cells.
17. Worked on the audio channels allocation priorities of the sound effects:
* given the Boltspitter bolt shooting and the hitting of the spaceship by the Mothership beam priority over the milestone notification;
* assigned the speech warnings specific priorities (from lowest to highest: "charge low", "shield low", "approaching asteroids", "escapes critical", "danger", "time running out").
18. Updated/improved the documentation.
19. Updated the artwork to reflect the new cells graphics.