sl0ppty: decode the PNGs programs actually send

Kitty graphics were green in the suite and a real program still drew
nothing. libghostty-vt ships no image codec: built as a library it sets
its `sys.decode_png` hook to null, and a null hook rejects every `f=100`
transmission outright -- no image stored, so no placement exists, so the
re-emission layer this project is mostly *about* is never reached. The
program gets no error and no picture.

Raw RGB worked throughout, which is why it looked done: every test here
sent raw pixels, because that is what is easy to write in a test. It is
the same lesson the last kitty graphics bug taught -- write the test the
way a program actually behaves -- and a program with a picture on disk
has a PNG. Found because ida-tui's startup logo never appeared, and its
detection probe answers `OK` first, so it fails silently rather than
falling back to its block art.

PNG is the whole of the gap and cannot grow: the protocol has exactly
three formats (`f=24`, `f=32`, `f=100`), the first two are raw pixels
lib-vt already handles, and `decode_png` is the only codec hook it has.
Decoding only, never encoding.

The decoder is the stb_image lib-vt already vendors and we already
commit -- upstream compiles that same header with the same
`STBI_ONLY_PNG` for this same purpose, it is just not in the
`-Demit-lib-vt` build. So nothing new is pulled in; a file we already
ship is finally compiled. src/png.c is the hook, a header pre-check and
a copy: dimensions are read before decoding because the buffer we return
is allocated through lib-vt's limited allocator but stb's own working
allocation is plain malloc and answers to nothing.

Tests cover a PNG placed at the size it asked for (RGB and RGBA sources,
both normalised to RGBA), the client being handed `f=32` rather than the
png it was given, and a malformed png placing nothing without taking the
pane with it. They fail on the previous binary.

1 files changed, 6 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 7174c5c..76df199 100644
--- a/Makefile
+++ b/Makefile
@@ -53,6 +53,12 @@ $(BIN): $(OBJ) $(VT_LIB)
 build/%.o: src/%.c build/version.h $(VT_LIB) | build
 	$(CC) $(CFLAGS) -c $< -o $@
 
+# stb_image comes with the vendored libghostty-vt, which is where the include
+# path points. It is third-party and does not build clean under our warnings,
+# so it gets its own rule rather than every file paying for it.
+build/png.o: src/png.c build/version.h $(VT_LIB) | build
+	$(CC) $(CFLAGS) -I$(VT)/src/stb -Wno-unused-function -c $< -o $@
+
 # Rewritten only when the value actually changes, so `make` after a commit
 # rebuilds, and `make` twice in a row does not.
 .PHONY: FORCE