Fix mixed color clusters
When rendering clusters that have colored and non-colored glyphs, we were falling back to using show_text_glyphs, rendering all glyphs in the cluster without color. Instead, make composite_one_color_glyph smart enough to handle non-color glyphs, and only skip clusters that are entirely non-color.
Testcase: hb-view AmiriQuranColored.otf -u 627,64e