patch 9.2.0863: MS-Windows GUI: window contents can be missing when VimEnter is slow

patch 9.2.0863: MS-Windows GUI: window contents can be missing when VimEnter is slow

Problem:  On MS-Windows the text area can stay undrawn while a slow VimEnter
          autocommand runs, so the window shows what was on the screen behind
          it. Whether this happens depends on when the first paint message
          arrives.
Solution: Draw the text area when the window is opened, instead of waiting for
          the message loop.

related: #20807
closes:  #20858

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>