examples: use cogl_framebuffer_clear4f not cogl_clear

cogl_clear depends on the default CoglContext which we are trying to
steer the API away from requiring. cogl_framebuffer_clear4f is
explicitly passed a framebuffer pointer which is implicitly related to a
specific context.

This updates all the examples to use cogl_framebuffer_clear4f instead of
cogl_clear and removes any redundant CoglColor that was previously
passed to cogl_clear.

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

examples/cogl-hello.c

@@ -37,7 +37,7 @@ main (int argc, char **argv)
         int n_poll_fds;
         gint64 timeout;
 
-        cogl_clear (&black, COGL_BUFFER_BIT_COLOR);
+        cogl_framebuffer_clear4f (fb, COGL_BUFFER_BIT_COLOR, 0, 0, 0, 1);
         cogl_primitive_draw (triangle);
         cogl_framebuffer_swap_buffers (fb);