Disable rendering on next listened to event after horizontal scroll

# Details

This plugin does not and will not gracefully handle horizontal
scrolling.

As such add logic to validation to check the left column of the
current window attempting to be rendered.

If this value is larger than zero rendering will not proceed.

We could add an additional event to listen to scroll events, like so:

```lua
vim.api.nvim_create_autocmd({ 'WinScrolled' }, {
    group = group,
    callback = function()
        for win, changes in pairs(vim.v.event) do
            if win ~= 'all' and changes.leftcol ~= 0 then
                local buf = util.win_to_buf(win)
                vim.schedule(function()
                    ui.refresh(buf)
                end)
            end
        end
    end,
})
```

Which would disable and re-enable rendering on horizontal scroll
events as appropriate.

However listening to every single scroll event seems heavy for this
plugin to do, so I would rather avoid it. Maybe make it opt-in, at
some point in the future if users want.

lua/render-markdown/ui.lua

@@ -57,10 +57,17 @@ M.clear_valid = function(buf)
         return false
     end
     vim.api.nvim_buf_clear_namespace(buf, M.namespace, 0, -1)
-    if util.buf_to_win(buf) < 0 then
+    local win = util.buf_to_win(buf)
+    if win < 0 then
         return false
     end
     util.set_conceal(buf, state.config.conceal.default)
+    local leftcol = vim.api.nvim_win_call(win, function()
+        return vim.fn.winsaveview().leftcol
+    end)
+    if leftcol > 0 then
+        return false
+    end
     return true
 end