During screen redraw, background window drawing would output characters
to the terminal for cells covered by popup windows. When the output
buffer flushed between background and popup drawing, the raw background
briefly appeared before the popup was redrawn, causing visible flicker
during mouse wheel scrolling.

Fix this in two ways:

1. In the skip_for_popup block in screen_line(), only call screen_char()
   during popup drawing (screen_zindex > 0). During background drawing
   (screen_zindex == 0), update the screen buffer but suppress terminal
   output. Set redraw_this = FALSE to prevent redundant drawing.

2. For opacity popups (which skip popup_mask), add a separate
   popup_opacity_mask array that tracks cells under opacity popups.
   Check this mask in screen_char() and screen_char_2() to suppress
   terminal output during background drawing. This covers all output
   paths including screen_fill().

# Conflicts:
#	src/screen.c

- Always call popup_adjust_position() in popup_setoptions() so that
  border, close, padding changes are properly applied (restores
  original behavior).
- Use lightweight redraw (w_redr_type) for content-only changes
  (firstline, highlights) to avoid flickering.
- Remove popup_opacity_mask: master's patch v9.2.0104 already handles
  background suppression for opaque popups via screen_opacity_popup,
  and popup_opacity_mask was incorrectly blocking terminal output for
  transparent popup cells.