Fix aaline width and blend conflict

[ankith26]

• blend got removed in 2.4.0
• blend was re-added in 2.5.0 as a no-op
• added a width implementation in 2.5.2 but reverted it for that release only when it should have also been reverted for all upcoming releases.
• 2.5.3, 2.5.4 and 2.5.5 all use the initial width implementation. The problem here is that the width argument is coming in place of the depth argument so we are still breaking compat.
• just got the new width implementation in for 2.5.6.dev1. Still have the same issue as the previous three releases.

I saw 2 potential fixes for this situation:
fix 1: make width keyword only, put it after the blend arg, announce the API as new in 2.5.6 and pretend nothing happened in 2.5.[3-5]
fix 2: roll with whatever we have right now. This somehow works out because old code doing blend=1 or blend=True would now mean width=1 which is equivalent. We just need to add compat code that handles width <= 0 like width=1. We can then advertise that the "width argument replaces the blend argument" and old code doesn't break too bad.