library: vendors/stb_image + vendors/stb_truetype — stb ships with sx

The stb headers move from the repo-root vendors/ (resolvable only
with CWD = sx repo) into library/vendors/ following the sqlite
convention — bindings module + c/ sources + provenance README — so
'#import "vendors/stb_image/stb_image.sx"' (image v2.30 + image_write
v1.16) and '#import "vendors/stb_truetype/stb_truetype.sx"' (v1.26)
work from any consumer via the stdlib search paths. modules/ffi/stb.sx
dissolves into the stb_image vendor; modules/ffi/stb_truetype.sx keeps
its non-stb text-shaping companions and re-imports the vendored unit.
examples/1625 pins a deterministic in-memory BMP decode; examples/1626
pins font init + metric invariants against the system Helvetica.

library/vendors/stb_image/README.md

@@ -0,0 +1,20 @@
+# vendors/stb_image — image decode/encode for sx programs
+
+- stb_image: **v2.30**; stb_image_write: **v1.16** (version comments at
+  the top of each header)
+- Source: <https://github.com/nothings/stb> (`stb_image.h`,
+  `stb_image_write.h`)
+- License: public domain / MIT, dual (see the headers' license blocks)
+- Files: `c/stb_image.h` + `c/stb_image_impl.c`, `c/stb_image_write.h`
+  + `c/stb_image_write_impl.c` (each impl .c defines the
+  `*_IMPLEMENTATION` macro and includes its header)
+
+`#import "vendors/stb_image/stb_image.sx"` resolves through the stdlib
+search paths; the decls (`stbi_load`, `stbi_load_from_memory`,
+`stbi_image_free`, `stbi_write_png`, …) are synthesized from the
+headers, and the implementation compiles once per machine through sx's
+object cache. `examples/1625-vendor-stb-image-decode.sx` pins an
+in-memory BMP decode in the sx suite.
+
+To upgrade: replace the headers under `c/` with newer upstream copies,
+update this file, and rebuild (the object cache keys on source bytes).

library/vendors/stb_image/c/stb_image_impl.c

@@ -0,0 +1,2 @@
+#define STB_IMAGE_IMPLEMENTATION
+#include "stb_image.h"

library/vendors/stb_image/c/stb_image_write_impl.c

@@ -0,0 +1,2 @@
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+#include "stb_image_write.h"

library/vendors/stb_image/stb_image.sx

@@ -0,0 +1,19 @@
+// =====================================================================
+// vendors/stb_image — Sean Barrett's stb_image (decode) and
+// stb_image_write (encode) as one `#import c` unit (c/, see README.md
+// for versions + provenance).
+//
+// `#import "vendors/stb_image/stb_image.sx"` gives any sx program
+// image decode/encode with no system dependency and no build flags:
+// the decls are synthesized from the headers, and the implementation
+// compiles through sx's content-addressed object cache — once per
+// machine, not once per run.
+// =====================================================================
+
+#import c {
+    #include "c/stb_image.h";
+    #source  "c/stb_image_impl.c";
+
+    #include "c/stb_image_write.h";
+    #source  "c/stb_image_write_impl.c";
+};