Add fixed-point math for newbies tutorial

[avivace]

Originally by sasha

[AntonioND]

Something like this would be clearer:

int player_speed = (1 << player_fp) + 5;

Or even something like this:

int player_speed = (int)((1 << player_fp) * 1.625);

However, for this, player_fp would need to be const.

In fact, I think that a section on how to generate the initial values from floating point values would be nice. You would need to explain that operations with constant numbers will be optimized by the compiler and that they aren't done at runtime.

[AntonioND]

Yeah, a section of converting between float and fixed point would be useful, explaining that this can have a huge impact if done at runtime, and explaining in which cases the compiler will do the conversion at compile time.

[avivace]

I'll merge this as it seems to be stale.. @AntonioND could you implement your suggestions and send another PR?