Added GcBitmapGraphics.BlendMode implementation. The specified blend mode affects all drawing (graphics, text and images).
Added BitmapRenderer.BackgroundBitmap property which specifies a bitmap providing background for all drawing operations.
Added BitmapRenderer.TransparencyMaskBitmap property which specifies a grayscale bitmap providing transparency mask for all drawing operations.
Added GcBitmap.StoreInTempFile property which indicates whether pixel data should be dynamically mapped to a temporary file rather than kept fully in memory.
Added IImage.ToGcBitmap(GcBitmap) method which retrieves the underlying GcBitmap, or creates a new GcBitmap that contains the image data.
