World: replace skybox changer with a PNG skybox image

sv_skyname did not take effect at runtime in this build, so the Skybox card now
replaces the skybox materials' textures with <config dir>/skybox.png.

The PNG is decoded through libpng16's simplified API (dlopen'ed, so no new link
dependency) and uploaded with IMaterialSystem::CreateTextureFromBits as an
RGBA8888 texture. IMaterialVar::SetTextureValue assigns it to $basetexture and
the HDR variants for every material in the SkyBox textures group (all six
faces get the same image).

Original textures are recorded per material/var (texture pointer plus string
fallback) and restored when the feature is off. Records survive level changes
so a reused sky material is still restorable; the override is re-applied to a
new map automatically. The card shows the image path and has a Reload image
button; a missing/unreadable PNG raises an error toast.

1 files changed, 2 insertions, 4 deletions

diff --git a/src/hl2h.h b/src/hl2h.h
index 3cb131d..fd7a863 100644
--- a/src/hl2h.h
+++ b/src/hl2h.h
@@ -219,10 +219,7 @@ extern bool g_tint_world_on, g_tint_sky_on, g_tint_prop_on;
 extern int g_tint_world_rgb[3], g_tint_sky_rgb[3], g_tint_prop_rgb[3];
 extern bool g_fog_skybox, g_fog_disable;
 extern int g_fog_rgb[3], g_fog_start, g_fog_end;
-extern int g_skybox_choice;
-int skybox_option_count();
-const char *skybox_option_name(int i);
-const char *const *skybox_options();
+const char *config_file_cstr(const char *name);
 extern bool g_bhop_autostrafe;
 extern bool g_jumpbug_assist;
 extern bool g_spread_no_spread;
@@ -267,6 +264,7 @@ void fog_update();
 void fog_invalidate();
 void skybox_update();
 void skybox_invalidate();
+void skybox_reload_image();
 bool resolve_server_netvars();
 int server_entity_health(int idx, int *maxhealth, int *lifestate, void **base_out);
 bool server_set_velocity_z(int idx, float vz);