gdiplus: Do not access Bitmap bits when drawing transparent pixels.

dlls/gdiplus/graphics.c

@@ -365,8 +365,12 @@ static GpStatus alpha_blend_bmp_pixels(GpGraphics *graphics, INT dst_x, INT dst_
         for (y=0; y<src_height; y++)
         {
             ARGB dst_color, src_color;
-            GdipBitmapGetPixel(dst_bitmap, x+dst_x, y+dst_y, &dst_color);
             src_color = ((ARGB*)(src + src_stride * y))[x];
+
+            if (!(src_color & 0xff000000))
+                continue;
+
+            GdipBitmapGetPixel(dst_bitmap, x+dst_x, y+dst_y, &dst_color);
             GdipBitmapSetPixel(dst_bitmap, x+dst_x, y+dst_y, color_over(dst_color, src_color));
         }
     }

dlls/gdiplus/tests/graphics.c

@@ -2417,6 +2417,27 @@ static void test_fromMemoryBitmap(void)
     GdipDeleteGraphics(graphics);
 
     GdipDisposeImage((GpImage*)bitmap);
+
+    /* If we don't draw to the HDC, the bits are never accessed */
+    status = GdipCreateBitmapFromScan0(4, 4, 12, PixelFormat24bppRGB, (BYTE*)1, &bitmap);
+    expect(Ok, status);
+
+    status = GdipGetImageGraphicsContext((GpImage*)bitmap, &graphics);
+    expect(Ok, status);
+
+    status = GdipGetDC(graphics, &hdc);
+    expect(Ok, status);
+    ok(hdc != NULL, "got NULL hdc\n");
+
+    color = GetPixel(hdc, 0, 0);
+    todo_wine expect(0x0c0b0d, color);
+
+    status = GdipReleaseDC(graphics, hdc);
+    expect(Ok, status);
+
+    GdipDeleteGraphics(graphics);
+
+    GdipDisposeImage((GpImage*)bitmap);
 }
 
 static void test_GdipIsVisiblePoint(void)