This is a really awesome function that I wanted to adapt for my own project. In doing so I wanted to make it draw out the inner corners and add a little more flexibility. My code's a little messy since rather than going from scratch to make the functions as clean as they could be I just retro-fit them into wookies, but hopefully shouldn't be a problem.
Additional to drawing in the inside corners, it also adds three variables which you'll need to initialize somewhere pre-draw for additional functionality. If you want to maintain wookie's standards, alpha would be 1, color would be c_dkgray and thickness is 1 which you can just place in the create function and adjust via creation code (or however) on an individual basis. Keep in mind that as with the normal setup, you'll need to free the surface if you want it to redraw after changing any of these values.
alpha -> simply affects the output opacity of the drawn surface, in case you want the seamless blocks to be semitransparent.
color -> lets you adjust the coloring of the block borders
thickness -> lets you adjust the thickness of the block borders
Should hopefully be able to just drop this into the draw code instead:Updated Code