Previous API worked under the assumption that download_image is always called
after map_image. In practice this is true, but it's better to have a much
generic API that doesn't depend on the order in which the functions are called.
The harder work was done in the previous commits. After that this feature comes
out almost for free.
The only problem is I can't get the textures created with CGLTexImageIOSurface2D
to download properly, thus the code performs download using some CoreVideo APIs.
If someone knows why download of textures created with CGLTexImageIOSurface2D
doesn't work please contact me :)