Do not load files with invalid paint mutators.

Fails importing files with multiple paint mutators per paint. This is invalid and the editor does clean this up. We also print a helpful message in debug mode so you can know why the file failed to load.

Details here: https://2dimensions.slack.com/archives/CLLCU09T6/p1729178961824429

Diffs=
96d45f6824 Do not load files with invalid paint mutators. (#8366)

Co-authored-by: Luigi Rosso <luigi-rosso@users.noreply.github.com>

.rive_head

@@ -1 +1 @@
-76f79a22e26bcc672840e3ba601cb84eb938aecb
+96d45f68240ef35a38c91eeec654ed771a006458

include/rive/core.hpp

@@ -5,6 +5,12 @@
 #include "rive/core/binary_reader.hpp"
 #include "rive/status_code.hpp"
 
+#ifdef DEBUG
+#define DEBUG_PRINT(msg) fprintf(stderr, msg "\n");
+#else
+#define DEBUG_PRINT(msg)
+#endif
+
 namespace rive
 {
 class CoreContext;

include/rive/shapes/paint/shape_paint_mutator.hpp

@@ -1,6 +1,8 @@
 #ifndef _RIVE_SHAPE_PAINT_MUTATOR_HPP_
 #define _RIVE_SHAPE_PAINT_MUTATOR_HPP_
 
+#include "rive/status_code.hpp"
+
 namespace rive
 {
 class Component;
@@ -16,7 +18,7 @@ class ShapePaintMutator
 protected:
     /// Hook up this paint mutator as the mutator for the shape paint
     /// expected to be the parent.
-    bool initPaintMutator(Component* component);
+    StatusCode initPaintMutator(Component* component);
     virtual void renderOpacityChanged() = 0;
 
     RenderPaint* renderPaint() const { return m_RenderPaint; }

src/shapes/paint/linear_gradient.cpp

@@ -20,11 +20,7 @@ StatusCode LinearGradient::onAddedDirty(CoreContext* context)
         return code;
     }
 
-    if (!initPaintMutator(this))
-    {
-        return StatusCode::MissingObject;
-    }
-    return StatusCode::Ok;
+    return initPaintMutator(this);
 }
 
 void LinearGradient::buildDependencies()

src/shapes/paint/shape_paint_mutator.cpp

@@ -4,20 +4,27 @@
 
 using namespace rive;
 
-bool ShapePaintMutator::initPaintMutator(Component* component)
+StatusCode ShapePaintMutator::initPaintMutator(Component* component)
 {
     auto parent = component->parent();
     m_Component = component;
     if (parent->is<ShapePaint>())
     {
+        if (parent->as<ShapePaint>()->renderPaint() != nullptr)
+        {
+            DEBUG_PRINT(
+                "ShapePaintMutator::initPaintMutator - ShapePaint has already "
+                "been asigned a mutator. Does this file have multiple solid "
+                "color/gradients in one single fill/stroke?");
+            return StatusCode::InvalidObject;
+        }
         // Set this object as the mutator for the shape paint and get a
         // reference to the paint we'll be mutating.
         m_RenderPaint = parent->as<ShapePaint>()->initRenderPaint(this);
-        return true;
+        return StatusCode::Ok;
     }
-    return false;
+    return StatusCode::MissingObject;
 }
-
 void ShapePaintMutator::renderOpacity(float value)
 {
     if (m_RenderOpacity == value)

src/shapes/paint/solid_color.cpp

@@ -12,12 +12,12 @@ StatusCode SolidColor::onAddedDirty(CoreContext* context)
     {
         return code;
     }
-    if (!initPaintMutator(this))
+    if ((code = initPaintMutator(this)) == StatusCode::Ok)
     {
-        return StatusCode::MissingObject;
+        renderOpacityChanged();
     }
-    renderOpacityChanged();
-    return StatusCode::Ok;
+
+    return code;
 }
 
 void SolidColor::renderOpacityChanged()