This removes the cogl_flush() api which depends on a global Cogl context
and provides a more specific _cogl_framebuffer_flush() instead. This
new function isn't public since we don't currently see a use for this
api for applications. (The previously documented use for cogl_flush() to
allow applications to intermix direct GL and cogl usage was missleading
and wouldn't have worked for intermixing drawing commands. Intermixing
GL to create objects such as textures doesn't require any flush api)

Since there are a few places internally where we do actually need to
flush *all* framebuffer journals we keep an equivalent to cogl_flush()
renamed to _cogl_flush() which now takes a CoglContext pointer.

cogl1-context.h has now been removed since there was nothing left in it.

Reviewed-by: Neil Roberts <neil@linux.intel.com>