DxRenderer should eat "concealed" text

# Description of the new feature/enhancement

Invisible text is generally rendered as text with a foreground color equal to the background color. This can be repro'd as a part of https://github.com/jdebp/terminal-tests/tree/master/PowerShell).

This makes it _not_ look invisible, however, because we apply the acrylic texture to the background, but not the foreground.

# Proposed technical implementation details (optional)

Just make the renderer _not_ render the invisible text. Ignore it completely.

Start by locating DxRenderer in the Windows Terminal codebase and reproduce the concealed-text case described using the terminal-tests PowerShell examples. Trace how text whose foreground matches the background is rendered, then verify that such text is omitted while visible text remains unchanged.

Written by the indexing model from the issue text.