Removed TextureContent._bitmap.

The importer still uses System.Drawing.Bitmap internally for loading / format conversion.
Font processing still uses bitmaps internally.
TextureProcessor will use a bitmap for 'resize power of two' operations.

However, they no longer assume there is a bitmap ready and identical to the actual TextureContent.Face[0][0], which was a source of errors. If they need a bitmap to do some isolated operation, they make one, then get rid of it.