Mark VBO as dirty when vertex colors change
Mark VBO as dirty when vertex colors change

This fixes a bug in which the GraphicsVertexBuffer class was
not marking its VBO as dirty when the vertex colors were changed
with the setColors() method.

Change-Id: I49a18b3c89654088672afe61eef076c76d770e1a

kylelutz/chemkit,kylelutz/chemkit,kylelutz/chemkit,kylelutz/chemkit