Many times bevel cannot satisfy all the user desires on bevel widths simultaneously. This patch adds a global width adjustment pass which tries to keep bevels as even as possible by starting with vertices that need to have a width adjustment, and visiting connected vertices from there, inheriting forced widths and setting other as you go.
Also fixed in here is the method for setting the offset when the 'percent' type is used, and dealing with some previously missed special cases where it was impossible to have two lines meet because of reflex angles.