Update fillCircleHelper (and related methods) to be consistent with drawCircleHelper

[BillMerryman]

The method fillCircleHelper is inconsistent with the drawCircleHelper method. The brief for fillCircleHelper suggests it can render quarter circle segments while only capable of rendering half circles, while the brief for drawCircleHelper inaccurately states it only supports two mask bits while actually supporting four, one for each quadrant.

This PR addresses that by modifying fillCircleHelper to draw each corner independently. This makes the fillCircleHelper implementation consistent with drawCircleHelper, at the cost of fillCircleHelper being slightly less efficient (four calls to writeFastVLine instead of two). The two dependent methods, fillCircle and fillRoundRect, have also been updated to correctly use this new implementation. The briefs for fillCircleHelper and drawCircleHelper have been updated to more accurately reflect their functionality.

[BillMerryman]

I closed this pull request to resubmit with an alternative approach.