HardwarePixelBuffer should be independent of Texture

[paroj]

just as HardwareVertexBuffer is independent of a Mesh. This will allow using them for staging data uploads to GPU which can be used for Texture Streaming in a pipeline.

currently a HardwarePixelBuffer only exists as part of Texture. As Textures do have no notion of shadowing, PixelBuffers are always unshadowed.

add explicit

• HardwarePixelBufferPtr Texture::download() -> UNPACK_BUFFER
• Texture::upload(HardwarePixelBufferPtr)
• HardwareBufferManager::createPixelBuffer -> PACK_BUFFER