Make backface culling be part of the legacy state

This adds an internal function to set the backface culling state on a
pipeline. This includes properties to set the culling mode (front,
back or both) and also to set which face is considered the front
(COGL_WINDING_CLOCKWISE or COGL_WINDING_COUNTER_CLOCKWISE). The actual
front face flushed to GL depends on whether we are rendering to an
offscreen buffer or not. This means that when changing between on- and
off- screen framebuffers it now checks whether the last flushed
pipeline has backface culling enabled and forces a reflush of the cull
face state if so.

The backface culling is now set on a pipeline as part of the legacy
state. This is important because some code in Cogl assumes it can
flush a temporary pipeline to revert to a known state, but previously
this wouldn't disable backface culling so things such as flushing the
clip stack could get confused.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-ext-functions.h

@@ -106,6 +106,8 @@ COGL_EXT_FUNCTION (void, glFlush,
                    (void))
 COGL_EXT_FUNCTION (void, glFrontFace,
                    (GLenum mode))
+COGL_EXT_FUNCTION (void, glCullFace,
+                   (GLenum mode))
 COGL_EXT_FUNCTION (void, glGenTextures,
                    (GLsizei n, GLuint* textures))
 COGL_EXT_FUNCTION (GLenum, glGetError,