I couldn't find a spec for this one on the internet, but this matches
the prototype in eglmesaext.h. The note in swap_region2 says:
4) How is this extension an improvement over EGL_NOK_swap_region?
RESOLVED: This extension builds on the previous
EGL_NOK_swap_region extension by requiring that the implementation
considers the update region provided by the application as a
mandate rather than a hint. This allows for region-restricted
rendering without requiring the use of preserved buffer swaps
which may be relatively expensive for the implementation.
Furthermore, whilst preserved swap behaviour is necessary for
incremental rendering, many applications fully re-render modified
surface regions and so don't require preserved swap
behaviour. This extension provides a lighter weight surface update
mechanism for such applications.
Eric Anholt