fix off-by-0.5 in convex hull calculation

Author: adroitwhiz

src/RenderWebGL.js

@@ -1807,12 +1807,12 @@ class RenderWebGL extends EventEmitter {
         let rr = -1;
         let Q;
         for (let y = 0; y < height; y++) {
-            _pixelPos[1] = y / height;
+            _pixelPos[1] = (y + 0.5) / height;
             // Scan from left to right, looking for a touchable spot in the
             // skin.
             let x = 0;
             for (; x < width; x++) {
-                _pixelPos[0] = x / width;
+                _pixelPos[0] = (x + 0.5) / width;
                 EffectTransform.transformPoint(drawable, _pixelPos, _effectPos);
                 if (drawable.skin.isTouchingLinear(_effectPos)) {
                     Q = [x, y];
@@ -1842,7 +1842,7 @@ class RenderWebGL extends EventEmitter {
             // Scan from right to left, looking for a touchable spot in the
             // skin.
             for (x = width - 1; x >= 0; x--) {
-                _pixelPos[0] = x / width;
+                _pixelPos[0] = (x + 0.5) / width;
                 EffectTransform.transformPoint(drawable, _pixelPos, _effectPos);
                 if (drawable.skin.isTouchingLinear(_effectPos)) {
                     Q = [x, y];