Previously, get_relative_line_num() did not account for window, which caused incorrect relative line numbers with multiwindow option.

Steps to reproduce

1. minimal.lua

vim.o.number = true
vim.o.relativenumber = true

vim.pack.add({
	{ src = "https://github.com/nvim-treesitter/nvim-treesitter" },
	{ src = "https://github.com/nvim-treesitter/nvim-treesitter-context" },
})

require("nvim-treesitter.configs").setup({
	ensure_installed = { "json" },
})

require("treesitter-context").setup({
	mode = "topline",
	multiwindow = true,
})

2. Make the context appear

3. Open a second window (e.g., with :sf, :copen, or <F1>).

Relative line numbers in the context become incorrect in the non-active window.

This fix ensures that the cursor line number is calculated per-window by passing the appropriate winid.