Some improvements to alpha blending in GDI

[bmuessig]

This does still not work, but it corrects a few bugs that would additionally hinder it from working. When moving the cursor over the gradient, it does now flicker and "appear" to display a gradient (probably just a redraw effect). At this point, I am not sure which component is not properly supporting the alpha channel and causing the gradient to not display properly. From what I gathered searching, it is probably 37-year-old GDI itself.