* clutter/cogl/cogl.h: Add cogl_fog_set() abstracting the
glFog() functions and enabling the GL_FOG flag.
* clutter/cogl/gl/cogl.c: Add GL implementation of cogl_fog_set().
* clutter/cogl/gles/cogl.c: Add GL/ES implementation of
cogl_fog_set().
* clutter.symbols: Add new symbols.
* clutter/clutter-stage.h: Add API to enable depth cueing on
the stage using a linear GL fog, and to set the parameters
for it (#637).
* clutter/clutter-stage.c (clutter_stage_paint): Enable the
GL fog if the ClutterStage:use-fog property is true.
* tests/test-depth.c: Test the new stage API.
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