CCNodeGradientRadial: Quick radial gradient using GL_TRIANGLE_FAN

CCNodeGradientRadial draws radial gradient just as
CCNodeGradient draws a linear gradient over a quad.

CCNodeGradientRadial offers a lot of control over how the gradient
should look using startColor, endColor and gradientFactor properties.

Performance wise it should be much faster than creating a radial
gradient using a fragment shader or using a texture. Also, since it
uses native gl draw call, it should save a lot in terms of assets.

At present CCNodeGradientRadial uses 14 vertex per quad to
create a gradient. But, it can be increased/decreased by changing the
value stored at macro kCCNodeGradientRadialResolution.

CCNodeGradientRadial is not an extension of CCNodeColor or
CCNodeGradient as might seem at first look. But interface-wise
it is very similar to CCNodeGradient.

The two main reasons are:

1. CCNodeColor only has 4 vertex available whereas for a
gradient we certainly need more than that.

2. CCNodeColor uses GL_TRIANGLE_STRIP for rendering whereas
CCNodeGradientRadial uses GL_TRIANGLE_FAN
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