sixel: performance improvements
* Store pointer to current pixel (i.e. pixel we're about to write to),
instead of a row-byte-offset. This way, we don't have to calculate the
offset into the backing image every time we emit a sixel band.
* Pass data pointer directly to sixel_add_*(), to avoid having to
calculate an offset into the backing image.
* Special case adding a single 1:1 sixel. This removes a for loop, and
simplifies state (position) updates. It is likely LTO does this for
us, but this way, we get it optimized in non-LTO builds as well.
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