[FairyGUI] Add support for blending

[rh101]

Describe your changes

DrawNode now inherits from BlendProtocol, since it does support blending. Also, ensure that the set BlendFunc is actually used, because it was hard-coded previously, so never used the correct blending modes. The change should not affect existing usage at all, since the default BlendFunc was BlendFunc::ALPHA_PREMULTIPLIED, which is {backend::BlendFactor::ONE, backend::BlendFactor::ONE_MINUS_SRC_ALPHA}, the same as what was being hard-coded:

blendDescriptor.sourceRGBBlendFactor        = backend::BlendFactor::ONE;
blendDescriptor.destinationRGBBlendFactor   = backend::BlendFactor::ONE_MINUS_SRC_ALPHA;
blendDescriptor.sourceAlphaBlendFactor      = backend::BlendFactor::ONE;
blendDescriptor.destinationAlphaBlendFactor = backend::BlendFactor::ONE_MINUS_SRC_ALPHA;

Which is now:

blendDescriptor.sourceRGBBlendFactor        = _blendFunc.src;
blendDescriptor.destinationRGBBlendFactor   = _blendFunc.dst;
blendDescriptor.sourceAlphaBlendFactor      = _blendFunc.src;
blendDescriptor.destinationAlphaBlendFactor = _blendFunc.dst;

Nothing else changes, as this is primarily an enhancement to allow blending when using FairyGUI components. The FairyGUI code changes are based of FairyGUI libraries for other engines, so the API is similar to their other implementations.

Issue ticket number and link

#2486

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