stb_truetype -- fix floating point comparison against zero by using a correct epsilon

stb_truetype.h

@@ -4547,6 +4547,9 @@ STBTT_DEF unsigned char * stbtt_GetGlyphSDF(const stbtt_fontinfo *info, float sc
    // invert for y-downwards bitmaps
    scale_y = -scale_y;
 
+   // distance from singular values (in the same units as the pixel grid)
+   const float eps = 1./128, eps2 = eps*eps;
+
    {
       int x,y,i,j;
       float *precompute;
@@ -4560,15 +4563,15 @@ STBTT_DEF unsigned char * stbtt_GetGlyphSDF(const stbtt_fontinfo *info, float sc
             float x0 = verts[i].x*scale_x, y0 = verts[i].y*scale_y;
             float x1 = verts[j].x*scale_x, y1 = verts[j].y*scale_y;
             float dist = (float) STBTT_sqrt((x1-x0)*(x1-x0) + (y1-y0)*(y1-y0));
-            precompute[i] = (dist == 0) ? 0.0f : 1.0f / dist;
+            precompute[i] = (dist < eps) ? 0.0f : 1.0f / dist;
          } else if (verts[i].type == STBTT_vcurve) {
             float x2 = verts[j].x *scale_x, y2 = verts[j].y *scale_y;
             float x1 = verts[i].cx*scale_x, y1 = verts[i].cy*scale_y;
             float x0 = verts[i].x *scale_x, y0 = verts[i].y *scale_y;
             float bx = x0 - 2*x1 + x2, by = y0 - 2*y1 + y2;
             float len2 = bx*bx + by*by;
-            if (len2 != 0.0f)
-               precompute[i] = 1.0f / (bx*bx + by*by);
+            if (len2 >= eps2)
+               precompute[i] = 1.0f / len2;
             else
                precompute[i] = 0.0f;
          } else
@@ -4633,8 +4636,8 @@ STBTT_DEF unsigned char * stbtt_GetGlyphSDF(const stbtt_fontinfo *info, float sc
                         float a = 3*(ax*bx + ay*by);
                         float b = 2*(ax*ax + ay*ay) + (mx*bx+my*by);
                         float c = mx*ax+my*ay;
-                        if (a == 0.0) { // if a is 0, it's linear
-                           if (b != 0.0) {
+                        if (fabs(a) < eps2) { // if a is 0, it's linear
+                           if (fabs(b) >= eps2) {
                               res[num++] = -c/b;
                            }
                         } else {