The 2 enum classes are identical except for the spelling of the values. DlBlendMode used the spelling that Skia used to use for ease of global replacement but now we have an unnecessary duplicate compared to the Impeller version. (Technically it is somewhat a dup of SkBlendMode as well, but we needed our own version for API isolation from Skia.)

We should replace the enum class DlBlendMode defined in DisplayList with just a using DlBlendMode = impeller::BlendMode statement and do the global name replacement to eliminate the need for "conversion" with Impeller.