ZX Spectrum isometric engine development

Or as known at the age, filmation engine. A set of routines useful for implementing different types of isometric engines will be developed tested and added to this repository.

This includes also routines to animate sprites with masks useful for othe types of games, and not only isometric.

There is not for now the objetive to develop any game with this routines, but only evaluate the behaviour of the code obtained (time results, memory footprint,..).