[cogl-texture] Fix offset to next quad when flushing the log

[cogl-texture] Fix offset to next quad when flushing the log

When the quad log contains multiple textures (such as when a sliced
texture is drawn) it dispatches the log with multiple calls to
flush_quad_batch and walks a pointer along the list of vertices.
However this pointer was being incremented by only one vertex so the
next quad would be drawn with three of the vertices from the last
quad.