Reworked border radius implementation

Thanks to @tobymckoi for the pointers to the w3c documentation on border radius I completely scrapped the previous implementation and made a much more well rounded implementation :wink:. Ignore the "maybe this time" commits, having never really used git before I made a complete blunder of my fork and those 2 commits are useless.

Changes are as follows:
 - Fixed overflow problem with border radius's that would would exceed 100% of a side
 - Fixed bug where inner border radius would not size down appropriately.
 - Cleaned up unused code for a more readable implementation
 - Performance improvement (70% faster)