EffectComposer: Add extraMargin property for generated effects

Extra margin property is added to effects that can spill outside the
source item. This property specifies the amount of space outside the
item that the effect is allowed to use for rendering.

Fixes: QDS-11607
Change-Id: I36d7392593faa6deb99726eaa02184aa87aa3571
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>

src/plugins/effectcomposer/compositionnode.h

@@ -52,6 +52,8 @@ public:
     int decRefCount();
     void setRefCount(int count);
 
+    int extraMargin() const { return m_extraMargin; }
+
 signals:
     void uniformsModelChanged();
     void isEnabledChanged();
@@ -70,6 +72,7 @@ private:
     QString m_id;
     bool m_isEnabled = true;
     int m_refCount = 0;
+    int m_extraMargin = 0;
 
     QList<Uniform *> m_uniforms;