Skybox image: decode PNG ourselves to avoid symbol interposition

client.so, engine.so and vguimatsurface.so all export their own
png_create_read_struct. Loading the system libpng16 made its internal calls
bind to the game's older libpng, whose png_struct layout differs, so
png_safe_warning read a NULL error_ptr and crashed.

Replace libpng with a small PNG reader: zlib's uncompress (dlopen'ed) plus the
five PNG row filters, supporting 8-bit non-interlaced RGB/RGBA/gray/palette
images with tRNS alpha for palettes. Output verified byte-identical to libpng
for the skybox.png in use.

1 files changed, 10 insertions, 5 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 669c1fb..5bd0436 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -363,11 +363,16 @@ vocabulary / decisions.
 - Replaces the map skybox with `<config dir>/skybox.png` (same directory as
   the INI). Not `sv_skyname`: that did not take effect at runtime in this
   build, so the skybox card now works on materials instead.
-- PNG is decoded with libpng16's simplified API (`dlopen`ed, no link
-  dependency). This build's `IMaterialSystem` interface ends at
-  `GetDisplayDeviceName` (the slots after it belong to `CMaterialSystem`
-  itself), so there is **no `CreateTextureFromBits`**. The pixels are uploaded
-  through `CreateProceduralTexture` (vtable 81) + an `ITextureRegenerator`
+- The PNG is decoded by our own reader (`zlib`'s `uncompress` + PNG filters;
+  8-bit non-interlaced RGB/RGBA/gray/palette). Do **not** use the system
+  libpng: `client.so`, `engine.so` and `vguimatsurface.so` each export their
+  own `png_create_read_struct`, so libpng16's internal PLT calls bind to the
+  game's older libpng with a different `png_struct` layout and crash in
+  `png_safe_warning` (error_ptr at +0xb0 reads NULL).
+- This build's `IMaterialSystem` interface ends at `GetDisplayDeviceName`
+  (the slots after it belong to `CMaterialSystem` itself), so there is **no
+  `CreateTextureFromBits`**. The pixels are uploaded through
+  `CreateProceduralTexture` (vtable 81) + an `ITextureRegenerator`
   (`SetTextureRegenerator` 12, `Download` 13), which fills mip 0 through
   `IVTFTexture::ImageData(frame,face,mip)` (39). `IVTFTexture`'s virtual
   destructor occupies two slots, so Width=15, Height=16, RowSizeInBytes=19.