Minor note about palette handling in encode

[jerch]

Here you state to reserve color slot 0 for transparent (aka background color):

go-sixel/sixel.go

Line 38 in 8aa94a3

nc := 255 // (>= 2, 8bit, index 0 is reserved for transparent key color)

Following the SIXEL spec in DEC STD 070 this is actually not needed, there is no link or notion in the spec of slot 0 being reserved for unencoded pixels. The addressable color slots are independent from the background color (thus you can use all 256 slots). The background color itself is something a device might provide by other means (prolly startup color or BCE).

[mattn]

Can you show me your fix?