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.

vendors/stb_image/stb_image_impl.c

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

vendors/stb_image/stb_image_write_impl.c

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

vendors/stb_truetype/stb_truetype_impl.c

@@ -1,2 +0,0 @@
-#define STB_TRUETYPE_IMPLEMENTATION
-#include "stb_truetype.h"