Convert the Transmission.Format, Transmission.Medium, and
Transmission.Compression types from plain Zig enums to lib.Enum so
they get a C-compatible backing type when building with c_abi. This
lets the C API layer reuse the types directly instead of maintaining
separate mirror enums.
Move Format.bpp() to a standalone Transmission.formatBpp() function
since lib.Enum types cannot have decls.
In the C API layer, rename kitty_gfx to kitty_storage and command to
kitty_cmd for clarity, and simplify the format/compression getters
to direct assignment now that the types are shared.
9ff4bb2df5