Use float literals for float calculations in Color and misc core cleanup

[aaronfranke]

This PR uses float calculations in Color and is also a continuation of #66962.

• Color:
  • Use float literals for float calculations and most other places using floats.
  • Only one of these changes actually changes behavior: the floats in srgb_to_linear
• Replace #include "core/string/print_string.h" with #include "core/string/ustring.h" where relevant, and in a few places remove #include "core/string/print_string.h" where it's not needed.
• In Projection::get_endpoints, put the planes we're using on their own lines. This is for consistency with godot-cpp since there it's a Variant array so we need Plane casts, but the constructor-style cast doesn't work in this case and I figured the line was getting long and complicated so I put it on its own line. I think it's more readable anyway.
• Some comment and whitespace changes.

[akien-mga]

Thanks!