Fix flickering caught in high level runtimes.

When converting floating point opacity values to 8 bit alpha, make sure to not under/overflow 0-255.

Fixes rive-app/rive-react-native#159

Diffs=
e65175b88 Fix flickering caught in high level runtimes. (#5243)

.rive_head

@@ -1 +1 @@
-a8c1e3e758819373ba9246b3731d395f89fa3575
+e65175b88afb40b101b54617a29b1bedc6f63d99

src/shapes/paint/color.cpp

@@ -41,19 +41,24 @@ void UnpackColorToRGBA32FPremul(ColorInt color, float out[4])
 
 float colorOpacity(ColorInt value) { return (float)colorAlpha(value) / 0xFF; }
 
+static uint8_t opacityToAlpha(float opacity)
+{
+    return (uint8_t)std::lround(255.f * std::max(0.0f, std::min(1.0f, opacity)));
+}
+
 ColorInt colorWithAlpha(ColorInt value, unsigned int a)
 {
     return colorARGB(a, colorRed(value), colorGreen(value), colorBlue(value));
 }
 
 ColorInt colorWithOpacity(ColorInt value, float opacity)
 {
-    return colorWithAlpha(value, std::lround(255.f * opacity));
+    return colorWithAlpha(value, opacityToAlpha(opacity));
 }
 
 ColorInt colorModulateOpacity(ColorInt value, float opacity)
 {
-    return colorWithAlpha(value, std::lround(255.f * colorOpacity(value) * opacity));
+    return colorWithAlpha(value, opacityToAlpha(colorOpacity(value) * opacity));
 }
 
 static unsigned int lerp(unsigned int a, unsigned int b, float mix)