Current implementation of SrcOver was done via multipliers of ColorAttachmentDescriptor (see https://developer.apple.com/documentation/metal/mtlrenderpipelinecolorattachmentdescriptor/1514657-destinationalphablendfactor?language=objc). Some of rules can't be implemented through this multipliers (for example, support of extra-alpha can be done only via shaders). Need to implement all composite rules via fragment shader.