gdiplus: GdipGraphicsClear() should overwrite pixels, not alpha blend.

gdiplus: GdipGraphicsClear() should overwrite pixels, not alpha blend.

All sample code in the Python Pyglet library suffers from terrible
text corruption, caused by the inability to erase the background
between rendering sequential font glyphs, resulting in leftovers
from previous letters mixing with the image of new letters.

This is because it attempts to erase the background by calling
GdipGraphicsClear() with ARGB color 0x00000000 (completely
transparent black), and in our gdiplus alpha blending that into
the background has no effect. It should be using
CompositeModeSourceCopy to overwrite the background with that brush
instead.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=30506
Signed-off-by: Damjan Jovanovic <damjan.jov@gmail.com>
Signed-off-by: Vincent Povirk <vincent@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>