GdkGLContext: Fix damage computation with buffer_age

GdkGLContext: Fix damage computation with buffer_age

As per the spec:

> The back buffer can
> either be reported as invalid (has an age of 0) or it may be
> reported to contain the contents from n frames prior to the
> current frame.

So a  buffer age of 1 means that the buffer was used in the last frame.
We were handling buffer_age==1 the same as buffer_age==0, i.e. we
returned the full damage for the surface.

[1] https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_buffer_age.txt