Skybox image: power-of-two resize; upload at FRAME_RENDER_START

- The material system rejects textures whose dimensions are not multiples of 4
  ("Image dimensions must be multiple of 4!"), so scale the decoded image to
  the next power of two (bilinear). 798x798 -> 1024x1024. The invalid VTF also
  crashed the renderer after the apply.
- Move skybox_update() out of the VGUI paint hook into the FRAME_RENDER_START
  FrameStageNotify stage. ITexture::Download only regenerates bits when the
  material system considers the frame render-safe; during paint it returned
  early, the regenerator never ran (no pixels), and a later upload of the
  empty VTF crashed.
- Log the regenerator entry so the log shows whether the upload path ran.

1 files changed, 7 insertions, 1 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index b07e407..5eca9f8 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -368,7 +368,9 @@ vocabulary / decisions.
   the INI). Not `sv_skyname`: that did not take effect at runtime in this
   build, so the skybox card now works on materials instead.
 - 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
+  8-bit non-interlaced RGB/RGBA/gray/palette) and scaled to the next power of
+  two, because the material system rejects textures whose dimensions are not
+  multiples of 4 ("Image dimensions must be multiple of 4!"). 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
@@ -394,6 +396,10 @@ vocabulary / decisions.
   string as a fallback) and restored when the feature is disabled. Records
   survive level changes so a reused sky material can still be restored; the
   override is re-applied to the new map automatically.
+- `skybox_update()` runs from the `FRAME_RENDER_START` FrameStageNotify hook,
+  not from the VGUI paint hook: `ITexture::Download` only regenerates texture
+  bits on a render-safe frame stage. Running it during paint skipped the
+  regenerator and left an empty VTF (upload warnings + crash).
 - The card shows the image path and has a Reload image button. A missing or
   unreadable PNG raises an error toast; the image is only re-read on Reload.
 - Saved as `[skybox] active/bind`.