Add blend filter support for blend modes which use "advanced" blend equations (listed below) that are supported by Skia/SVG/PDF.

    kScreen,
    kOverlay,
    kDarken,
    kLighten,
    kColorDodge,
    kColorBurn,
    kHardLight,
    kSoftLight,
    kDifference,
    kExclusion,
    kMultiply,
    kHSLHue,
    kHSLSaturation,
    kHSLColor,
    kHSLLuminosity,

• These need fragment shaders.
• When we build out the GLES 2 and Vulkan backends, we can use GL_KHR_blend_equation_advanced and VK_EXT_blend_operation_advanced respectively on supporting devices to cover all of these.