DxRenderer should eat "concealed" text

[carlos-zamora]

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.

[j4james]

This should perhaps be independent of the concealed attribute though. If someone sets the foreground color to exactly match the background, I would think they'd also expect it to be invisible regardless of acrylic mode wouldn't they?

Also, don't forget about gridlines.

[DHowett]

Huh. Grid lines . . . good catch 😄

[DHowett]

Triaged into Terminal v2 -- help wanted. Thanks for all the input here!