sl0ppty: a pane can set its own shaders

    printf '\033]5577;1;shader;chrome;tint color="#ff5fd7" amount="abs(t / 8 % 510 - 255)"\033\\'

The field after `shader` is which rect -- `content` or `chrome` -- and the
rest of the payload is a chain in exactly the syntax a `shaders { }` block
uses, taken verbatim so that `;` can go on separating entries. Empty text
clears that chain. The session answers on the program's stdin, `shader-reply;ok`
or `shader-reply;error;bad amount for tint: ...`, because a prototyping loop
that gets no answer cannot tell a typo from a build without the feature.

`contrib/shader-repl` is the prompt this exists for: type a chain, watch the
pane, and `:paste` prints what you have as a config block. Both ends are the
same parser, so what you arrive at is what you can keep -- verified by
pasting `:paste`'s output into a file, linting it, and checking the frame
comes out the same colour.

**D13 said this was impossible on purpose, and the reasoning stands**: a
program that can restyle the session it happens to be running in is a hazard,
and `cat hostile.txt` should not dim your panes. But *impossible* was the
wrong shape for it -- editing a file, saving, looking and guessing again is
slow enough that shaders go unwritten. So it takes `in_band_shaders true`,
one line of consent, and it is scoped to the pane that asked: its own two
chains, never its neighbour's, never what the config said about anyone else.
There is a test for the neighbour.

Each chain owns the programs its expressions compiled to, and is swapped in
whole, so a chain that turns out not to read leaves the pane looking like it
did rather than half-restyled. Measured: 1,800 replacements of an animated
chain, resident size flat after the first pass.

Also here, because it is the same paint order: the chrome chain now sits
between the config's and policy's, exactly where the content chain does.
Dead code out -- `app_shade_add`/`app_shade_clear`/`app_shade_count` had no
callers and their comment claimed the opposite of what this commit does.

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/config.md b/docs/config.md
index b504111..57b816c 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -104,7 +104,7 @@ theme { frame_focus "#00ff88" }     // ...but that one colour is mine
 |---|---|
 | geometry | `gap` `gap_aspect` `padding` `rounded` `title_align` `title_inset` `min_pane` `min_split` |
 | chrome | `status_bar` `status_line` `status_pad` `hints` `version_banner` `pane_buttons` and the marks (`zoom_mark` `zoom_on_mark` `close_mark` `min_mark` `newtab_mark` `bell_mark`) |
-| behaviour | `focus_follows_mouse` `scroll_lines` `toast_ms` `hover_delay_ms` `double_click_ms` `anim_ms` `modal_scrim` `dim_unfocused` `keep_dead` `shell` `editor` `shader_dir` |
+| behaviour | `focus_follows_mouse` `scroll_lines` `toast_ms` `hover_delay_ms` `double_click_ms` `anim_ms` `modal_scrim` `dim_unfocused` `keep_dead` `in_band_shaders` `shell` `editor` `shader_dir` |
 | colour | `theme { }` |
 | effects | `shaders { }`, `states { }` — see [shaders](shaders.md) and [chrome](chrome.md) |
 | keys | `keys { }` — see [keys](keys.md) |