add getcenter functions to vertexdrawable
add getcenter functions to vertexdrawable

these are used for deciding where edges go within the layout engine, note the
drawing layer should probably draw vertices on top of edges such that these are covered

Signed-off-by: Sam Phippen <e5a09176608998a68778e11d5021c871e8fae93c@googlemail.com>

samphippen/spe,samphippen/spe,samphippen/spe,samphippen/spe