These sequences are used to operate with sixels, but they are still
str sequences, so they are finished with \a, ST or with a C1 control
code. This patch also disables utf8 handling for the case of sixels.
There are some ocasions where we want to disable the enconding/decoding of utf8, mainly
because it adds an important overhead. This is partial patch for ESC % G and ESC % @,
where they modified the way that st reads and write from/to the serial line, but it does
not modifies how it interacts with the X window part.
Roberto E. Vargas Caballero