Refactor: Restructured GL_PrepareTexture() (SOC)

This function has been reorganized so as to cleanly separate the
distinct phases of preparing an image for uploading to GL as a new
texture.
Refactor: Split texturecontent_t configuration from GL_PrepareTexture()
GL Texture Manager: Cleanup
Refactor: Moved GL_TexturePrepare() to Texture::Variant::prepare()

Functionally this logic is unchanged however it now operates within
the domain of Texture::Variant.
TextureVariant: Cleanup
Texture: Notify observers of a DimensionsChange
Merge branch 'master' of github.com:skyjake/Doomsday-Engine