btile: animated background btiles

[jorgegv]

Allow for animated btiles.

• Intended mainly for short and sporadic animation sequences
• This does not replaces sprites, btiles do not move
• An animated btile has N frames
• An animated btile has an animation sequence, which repeats over and over, governed by a repetition delay
• An animation sequence is a sequence of frame numbers (minimum 2, maximum 16), governed by animation delay
• This would very well benefit from #85 since there is the potential of reusing several 8x8 tiles between BTILE frames
• This would also benefit from #106 for better efficiency
• Reuse the sprite animation sequence code

Tasks:

• [x] Factor out sprite animation data structure for reuse
• [x] Factor out code for animation sequence state-machine for reuse (enemy.c)
• [ ] DATAGEN: add support for defining btiles with multiple frames (special case: normal btile with 1 frame)
• [ ] Add a new table to a screen which contains the animated btiles (btiles with a frame count > 1 and an animation sequence with >1 steps). Since the number of btiles in a screen can be big, the list needs to be precomputed.
• [ ] Modify game loop main function to call btile_animate_all function, which animates only animated btiles, extracted from the table indicated in the previous step
• [ ] Conditional compilation: btile definition with more than 1 frame activates build feature ANIMATED_BTILES
• [ ] Code related to animated btiles must be conditionally compiled wrt that build feature

Notes:

• According to https://worldofspectrum.org/forums/discussion/11729/ann-sp1-for-z88dk-aka-splib3-sprite-pack-v3-0/p2 , function sp1_PutTilesInv can be used for better efficiency.
• This function needs the data in a specific format (struct sp1_tp), but I think we can rearrange the BTile data layout to match that, and use that function instead.
• The point of that function is that the loop for all the N cells of a BTile is done fully in assembler, so it would be just one call to sp1_PutTilesInv instead of N calls to sp1_PrintAtInv. Also, calculations for indexes would be saved.