Use Math_TAU and deg2rad/etc in more places and optimize code

[aaronfranke]

This PR has many related changes to many parts of the code that work with angles or turns.

Floating-point math is not guaranteed to be associative, so in order for these pieces of code to be optimized, you either need to include parenthesis, calculate things in advance, or use -ffast-math, or else the compiler can't optimize these. Godot does not use that compile option, so we need to be explicit to have better performance.

• Pre-calculate step sizes in many places where for loops are used (Ctrl+F -> _step). This will improve performance quite a bit, since the code is executed many times in a row. It also improves readability in my opinion, since the old code had very complex lines.

• Add parenthesis to Math::deg2rad and Math::rad2deg around the constants. This allows the compiler to optimize this to a single multiplication (it can't do this automatically unless we use -ffast-math).

• Use Math::deg2rad and Math::rad2deg where applicable instead of * Math_PI / 180 etc. This should reduce code duplication, improve readability, and improve performance (just adding parenthesis would improve the performance, but using the method improves readability at the same time).

• Replace many occurrences of Math_PI multiplied by 2 with Math_TAU. This improves readability (including by avoiding parenthesis).

• Optimize some very ugly code in editor/node_3d_editor_gizmos.cpp at line 1074.

• Group constant calculations together. This improves performance and readability.

• Remove some unnecessary casts.

[reduz]

I am not entirely sure if I like this, I believe it makes code more confusing. Could be discussed at a PR meeting when they come back.

[Duroxxigar]

Can you elaborate on any area that makes the code more confusing? @reduz

For me, I don't see either side more or less confusing to read. Both versions are easy to read.

[AndreaCatania]

Looks fine.

[akien-mga]

Thanks!