Don't let neighboring radii in GrAAFillRRectOp get too close

Since each corner uses a different reference point, it's possible that
FP rounding can cause the edges of very close radii on a common edge
to accidentally overlap. This can cause a pixel to be drawn/blended
more than once. This CL addresses the issue by not allowing
neighboring radii on a common edge to get closer than 1/16 pixel from
one another.

Bug: skia:8562
Change-Id: Ifda97c9a4c3973405f86f7fc6846a4073b3ab581
Reviewed-on: https://skia-review.googlesource.com/c/173036
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>

Author: csmartdalton86

src/gpu/ops/GrAAFillRRectOp.cpp

@@ -325,8 +325,10 @@ class GrAAFillRRectOp::Processor::Impl : public GrGLSLGeometryProcessor {
         v->codeAppend("float2 aa_bloatradius = axiswidths * pixellength * .5;");
 
         // Identify our radii.
-        v->codeAppend("float2 radii = float2(dot(radii_selector, radii_x), "
-                                            "dot(radii_selector, radii_y));");
+        v->codeAppend("float4 radii_and_neighbors = radii_selector"
+                              "* float4x4(radii_x, radii_y, radii_x.yxwz, radii_y.wzyx);");
+        v->codeAppend("float2 radii = radii_and_neighbors.xy;");
+        v->codeAppend("float2 neighbor_radii = radii_and_neighbors.zw;");
 
         v->codeAppend("if (any(greaterThan(aa_bloatradius, float2(1)))) {");
                           // The rrect is more narrow than an AA coverage ramp. We can't draw as-is
@@ -347,11 +349,14 @@ class GrAAFillRRectOp::Processor::Impl : public GrGLSLGeometryProcessor {
         v->codeAppend(    "radius_outset = floor(abs(radius_outset)) * radius_outset;");
         v->codeAppend(    "is_linear_coverage = 1;");
         v->codeAppend("} else {");
-                          // Don't let actual arc radii get smaller than a pixel.
+                          // Don't let radii get smaller than a pixel.
         v->codeAppend(    "radii = clamp(radii, pixellength, 2 - pixellength);");
+        v->codeAppend(    "neighbor_radii = clamp(neighbor_radii, pixellength, 2 - pixellength);");
+                          // Don't let neighboring radii get closer together than 1/16 pixel.
+        v->codeAppend(    "float2 spacing = 2 - radii - neighbor_radii;");
+        v->codeAppend(    "float2 extra_pad = max(pixellength * .0625 - spacing, float2(0));");
+        v->codeAppend(    "radii -= extra_pad * .5;");
         v->codeAppend("}");
-        // Bias radii slightly inward to avoid accidental overlap of geometries from fp rounding.
-        v->codeAppend("radii -= aa_bloatradius * 1e-3;");
 
         // Find our vertex position, adjusted for radii and bloated for AA. Our rect is drawn in
         // normalized [-1,-1,+1,+1] space.