Problem:    Slow redrawing with DirectX.
Solution:   Avoid calling gui_mch_flush() unnecessarily, especially when
            updating the cursor. (Ken Takata, closes #2560)

Problem:    MS-Windows: drawing underline, curl and strike-throw is slow,
            mFallbackDC not properly updated.
Solution:   Several performance improvements. (Ken Takata, Taro Muraoka,
            Yasuhiro Matsumoto, closes #2401)

Problem:    MS-Windows: does not show colored emojis.
Solution:   Implement colored emojis. Improve drawing speed. Make 'taamode'
            work. (Taro Muraoka, Yasuhiro Matsumoto, Ken Takata, close #2375)

Problem:    Text drawing on newer MS-Windows systems is suboptimal.  Some
            multi-byte characters are not displayed, even though the same font
            in Notepad can display them. (Srinath Avadhanula)
Solution:   Add the 'renderoptions' option to enable Direct-X drawing. (Taro
            Muraoka)