Implemented several methods necessary for copying between bitmaps. Note: only copying between two PixelBitmapContent<T> where T is the same type has been implemented.
Added missing implementation for BitmapContent.ToString()
GraphicsUtil.CompressTexture no longer throws an exception for non-pow2 dxt textures if the GraphicsProfile is HiDef (this matches original XNA behavior).
TextureProcessor tries to set its pixel data to _bitmap. Bitmap only exists if the texture was loaded by the TextureImporter and even in that situation the pixeldata was already setup.
Merge pull request #2527 from SickheadGames/SpriteSheetProcessorFixes

BitmapContent.Copy, PixelBitmapContent.CopyTo