Fix sixel filler character (#1422)

The sixel implementation uses the Braille pattern blank character
(U+2800) as a filler character, which is problematic because U+2800
is not a proper Unicode whitespace character. This fix changes the
filler character to a proper Unicode whitespace character U+2000.

Quote from https://en.wikipedia.org/wiki/Whitespace_character:
"The Braille Patterns Unicode block contains U+2800 BRAILLE PATTERN
BLANK, a Braille pattern with no dots raised. Some fonts display the
character as a fixed-width blank, however the Unicode standard
explicitly states that it does not act as a space."

sixel.go

@@ -9,7 +9,7 @@ import (
 
 const (
 	gSixelBegin  = "\033P"
-	gSixelFiller = '\u2800'
+	gSixelFiller = '\u2000'
 )
 
 type sixelScreen struct {