fix(mouse): do not fetch clipboard twice when pasting with middle button #33494

Problem:
When doing paste operation mouse code tries to figure out it it is
dealing with a multi-line register by calling yank_register_mline(),
which fetches register data and checks its type. Later the code calls
either do_put() or insert_reg() which fetch register data again. This is
unnoticeable when working with internal neovim registers, but starts
hurting when dealing with clipboards, especially remote one (forwarded X
or socket tunnel or similar).

Solution:
Change yank_register_mline() to also return pointer to the
register structure prepared for pasting, and insert_reg() to accept
such register pointer and use it if it is supplied. do_put() already
has support for accepting a register structure to be used for pasting.

Fixes #33493

src/nvim/edit.c

@@ -3250,7 +3250,7 @@ static void ins_reg(void)
 
       do_put(regname, NULL, BACKWARD, 1,
              (literally == Ctrl_P ? PUT_FIXINDENT : 0) | PUT_CURSEND);
-    } else if (insert_reg(regname, literally) == FAIL) {
+    } else if (insert_reg(regname, NULL, literally) == FAIL) {
       vim_beep(kOptBoFlagRegister);
       need_redraw = true;  // remove the '"'
     } else if (stop_insert_mode) {