Removed TextureContent._bitmap.
It was only used in two locations:
1. Resizing a texture (for the resize-pow-two condition in TextureProcessor). It allocates a bitmap and then disposes it when done with this operation.
2. Font processor works with bitmaps, but rather than assuming there is one on the texturecontent up front, it just creates one.

This means that everyone else in the pipeline can rely on TextureContent.GetData/SetData always being what you expect it to be instead of being mysteriously out of sync with a _bitmap that your custom processor doesn't know about or have access to.