Some enhancements and bug fixes

-Hack to align "to the whole pixel" in WebGL 'buildLine" causes unwanted offset when lineWidth is not an integer, what is the point ?
-Firefox is not able to render correctly massive TilingSprite with big scale and large off-screen rendering (accuracy issue), changed to recompute the filled rect as small as possible around the rendering zone.
-Added method drawArc with begin/end angle