fix: radialGradient r={0} (#2271)

# Summary

* Fixed crash on Android when `<RadialGradient r={0}>`.
* Fixed render issue on Android and iOS when radius is zero. According
to [MDN
Docs](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/r#radialgradient)
> A value of lower or equal to zero will cause the area to be painted as
a single color using the color and opacity of the last gradient
`<stop>`.

## Test Plan

Test is available in `TestsExample` as `Test2170`

## Compatibility

| OS      | Implemented |
| ------- | :---------: |
| iOS     |    ✅    |
| Android |    ✅    |


Fixes #2170

TestsExample/src/Test2170.tsx

@@ -0,0 +1,65 @@
+import * as React from 'react';
+import {View} from 'react-native';
+import {
+  Circle,
+  RadialGradient,
+  Rect,
+  Stop,
+  Svg,
+  SvgXml,
+} from 'react-native-svg';
+
+// https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/r#example
+const svgXml = `
+<svg viewBox="0 0 300 200" xmlns="http://www.w3.org/2000/svg">
+  <radialGradient r="0" id="myGradient000">
+    <stop offset="0" stop-color="white" />
+    <stop offset="100%" stop-color="black" />
+  </radialGradient>
+  <radialGradient r="50%" id="myGradient050">
+    <stop offset="0" stop-color="white" />
+    <stop offset="100%" stop-color="black" />
+  </radialGradient>
+  <radialGradient r="100%" id="myGradient100">
+    <stop offset="0" stop-color="white" />
+    <stop offset="100%" stop-color="black" />
+  </radialGradient>
+
+  <circle cx="50" cy="50" r="0" />
+  <circle cx="150" cy="50" r="25" />
+  <circle cx="250" cy="50" r="50" />
+
+  <rect x="20" y="120" width="60" height="60" fill="url(#myGradient000)" />
+  <rect x="120" y="120" width="60" height="60" fill="url(#myGradient050)" />
+  <rect x="220" y="120" width="60" height="60" fill="url(#myGradient100)" />
+</svg>
+`;
+
+export default function App() {
+  return (
+    <View style={{flex: 1, justifyContent: 'center', alignItems: 'center'}}>
+      <SvgXml width={400} height={300} xml={svgXml} />
+      <Svg width={400} height={300}>
+        <RadialGradient id="a" r={0}>
+          <Stop offset={0} stopColor="#f00" />
+          <Stop offset="50%" stopColor="#0f0" />
+          <Stop offset="100%" stopColor="#00f" />
+        </RadialGradient>
+        <RadialGradient id="b" r="50%">
+          <Stop offset={0} stopColor="#f00" />
+          <Stop offset="100%" stopColor="#00f" />
+        </RadialGradient>
+        <RadialGradient id="c" r="100%">
+          <Stop offset={0} stopColor="#f00" />
+          <Stop offset="100%" stopColor="#00f" />
+        </RadialGradient>
+        <Rect x={0} y={100} width={100} height={100} fill="url(#a)" />
+        <Rect x={150} y={100} width={100} height={100} fill="url(#b)" />
+        <Rect x={300} y={100} width={100} height={100} fill="url(#c)" />
+        <Circle cx={50} cy={250} r={50} fill="url(#a)" />
+        <Circle cx={200} cy={250} r={50} fill="url(#b)" />
+        <Circle cx={350} cy={250} r={50} fill="url(#c)" />
+      </Svg>
+    </View>
+  );
+}

android/src/main/java/com/horcrux/svg/Brush.java

@@ -204,6 +204,14 @@ class Brush {
       double rx = getVal(mPoints[2], width, scale, textSize);
       double ry = getVal(mPoints[3], height, scale, textSize);
 
+      if (rx <= 0 || ry <= 0) {
+        // Gradient with radius = 0 should be rendered as solid color of the last stop
+        rx = width;
+        ry = height;
+        stops = new float[] {stops[0], stops[stops.length - 1]};
+        stopsColors =
+            new int[] {stopsColors[stopsColors.length - 1], stopsColors[stopsColors.length - 1]};
+      }
       double ratio = ry / rx;
 
       double cx = getVal(mPoints[4], width, scale, textSize) + offsetX;

apple/Brushes/RNSVGPainter.mm

@@ -232,6 +232,15 @@ void PatternFunction(void *info, CGContextRef context)
   CGFloat rx = [self getVal:[_points objectAtIndex:2] relative:width];
   CGFloat ry = [self getVal:[_points objectAtIndex:3] relative:height];
 
+  if (rx <= 0 || ry <= 0) {
+    // Gradient with radius = 0 should be rendered as solid color of the last stop
+    rx = width;
+    ry = height;
+    CGGradientRelease(gradient);
+    NSArray<NSNumber *> *gradientArray = @[_colors.firstObject, _colors.lastObject, _colors[_colors.count-2], _colors.lastObject];
+    gradient = CGGradientRetain([RCTConvert RNSVGCGGradient:gradientArray]);
+  }
+    
   double ratio = ry / rx;
 
   CGFloat fx = [self getVal:[_points objectAtIndex:0] relative:width] + offsetX;