gdiplus: how should GdipGraphicsClear work ?

[Rafał Mużyło]

This isn't marked as a patch, cause it's most likely incorrect in some
way. Yet, it also improves the results in some cases.

Namely, it seems that - contrary to the current state - clears with a
color with a = 0 shouldn't be no-ops. I'm not sure what color should be
used in such case; I'm using opaque black, cause neither transparent
black nor transparent white give proper results.

Obviously, a proper testcase is required.

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