CanvasRenderer: Add Gradient, LinearGradient, RadialGradient classes

Add Gradient, LinearGradient, and RadialGradient classes that
implement Linear and Radial Gradient features.
Each Gradient can be applied to Fill and Stroke of Shape after
setting Bounds and ColorStop suitable for style.

ex)
//Radial Gradient with Fill color
Dali::CanvasRenderer::RadialGradient radial = Dali::CanvasRenderer::RadialGradient::New();
Dali::Vector<Dali::CanvasRenderer::Gradient::ColorStop> stops;
stops.PushBack( {0.0f, Vector4(1.0f, 0.0f, 1.0f, 1.0f)} );
stops.PushBack( {1.0f, Vector4(0.5f, 1.0f, 0.0f, 1.0f)} );

radial.SetBounds(Vector2(110, 100), 50);
radial.SetColorStops(stops);
shape.SetFillGradient(radial);

//Linear Gradient with Stroke color
Dali::CanvasRenderer::LinearGradient linear = Dali::CanvasRenderer::LinearGradient::New();
Dali::Vector<Dali::CanvasRenderer::Gradient::ColorStop> stops;
stops.PushBack( {0.0f, Vector4(1.0f, 0.0f, 1.0f, 1.0f)} );
stops.PushBack( {1.0f, Vector4(0.5f, 1.0f, 0.0f, 1.0f)} );

linear.SetBounds(Vector2(10, 10), Vector2(100, 100));
linear.SetColorStops(stops);
shape.SetStrokeGradient(linear);

Change-Id: I747614bf1c6bcd59f30a74a23c4001cfdfe19d1a