Render circular bed when enabling bed at center

Enabled commented code for circular bed and tied it to "Machine center 0,0" setting.
Changed the commented code to use 32 sides rather than 16
Changed commented code to allow oval bed
Changed alternating color walls of build area to solid when using circular bed (it hurt the eyes)

I wasn't sure if this had been disabled because it just had not been implemented yet, or it wasn't wanted at all. I'm guessing this PR will give me the answer.