Some terminals (foot makes this claim explicitly) can draw
sixels more quickly when P2 is 0 than when P2 is 1. Since
we're already detecting transparency for the TAM matrix, do
the same globally, and prepare a sixel_p2_e value-result
parameter based on the results. If there are no transparent
pixels, emit P2=0 #1527.
At least mlterm unhides the cursor after emitting a
Sixel, even if it was hidden beforehand. Track this
behavior using 'sprixel_cursor_hack' in the tinfo cache.
Set this based on an "mlterm" TERM heuristic match.
When it is set, supply the 'civis' capability as
cursor_hack in blitterargs, and emit it at the end of
the sixel in sixel_blit() #1524.
Now that we explicitly set P2=1, we can safely emit a "Set Raster
Attributes" command, to tell the terminal the final size of the image
up front.
Also fix the ‘pad’ parameter (horizontal aspect ratio); XTerm rejects
sixels with pan or pad set to 0.
This indicates that empty pixels should "remain at their current
color". I.e. it makes them transparent.
This is in contrast to P2=0|2, where empty pixels are filled with the
"current background color" (which is either sixel color register #0,
or the current ANSI background color, depending on terminal and its
sixel implementation).
Note that due to what is most likely a bug, XTerm will behave as if
P2=1 if P2 is either left unset, or explicitly set to 0 or 2, as long
as we do *not* emit a "Set Raster Attributes" command.
- separate the methods into its own module.
- add constants NCVISUAL_OPTION_ADDALPHA, NCVISUAL_OPTION_HORALIGNED, NCVISUAL_OPTION_VERALIGNED
- add NcRgba type.
- update the docs.
