positions
Return result depends on how blob was constructed.
makeFromPosH returns 1 float per glyph (x pos)
makeFromPos returns 2 floats per glyph (x, y pos)
makeFromRSXform returns 4 floats per glyph
Blobs constructed by TextBlobBuilderRunHandler/Shaper default handler have 2 floats per glyph.
Return
glyph positions for the blob if it was made with makeFromPos, null otherwise