Previously, we changed borders to be drawn after content, so they are no longer
part of the same quads as the background. In our change, we gave the background
quad a transparent black border and the border quads transparent black
backgrounds. However, this caused the other channels to blend toward that black
color before becoming fully transparent, causing them to become darker.

In this PR, I source the "placeholder" color by duplicating the values for
the other channels and only adjust the alpha down to zero.