The lifetime of the journal VBO is entirely within the scope of the
cogl_journal_flush function so there is no need to store it globally
in the Cogl context. Instead, upload_vertices_to_vbo just returns the
new VBO. cogl_journal_flush stores this in a local variable and
destroys it before returning.
This also fixes an assertion when using the GLES backend which was
caused by nothing initialising the journal_vbo variable.
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