Eliminate gray border in ToGdipBitmap

[viskin]

Current implementation of ToGdipBitmap extension method (when PixelMode.Gray) keeps gray border around letters:

It doesn't look good when bright text is rendered on bright background. Instead, I propose to eliminate this background and use only alpha component for antialiasing. The PR produces following text:

Black text on white background produces same pixels when the PR applied.

[HinTak]

I agree that the rendered image looks wrong - but I wonder if this should be reported upstream? (sorry, used the wrong account).