In Sixel, we map the alpha value to either 0 or 255, as
it has no alpha concept. Kitty can freely reproduce eight
bits of alpha, so go ahead and use them. Improves image
quality of translucent bitmaps in Kitty at the expense of
some behavioral divergence depending on bitmap backend.
Sixel: detect SPRIXCELL_TRANSPARENT cells and mark them as such.
sprixel_invalidate: fix copy-and-paste, want both TRANSPARENT and
ANNIHILATED to block progression to INVALIDATED. add gigantic
comment detailing the state machine. #1537, #1522, #1527, #1483.
When the damaged cell of a sprixel is actually entirely
transparent (or annihilated), there's no need to invalidate
it. Check for this in the TAM in sprixel_invalidate(). Good
optimization, and eliminates a lot of the flicker in `xray`
on Kitty described in #1522.
cell_release() and cell_duplicate() were deprecated in 2.2.6,
but replaced with static inlines. this breaks the abi, forcing
recompiles from client programs. restore them as exported
functions.
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.
