Added deps/Findstb.cmake (fetches stb via FetchContent, provides
stb::stb interface target).
Linked to cbt_opengl. Implemented window::screenshot() using
glReadPixels + vertical flip + stb_image_write to save RGBA PNG.
Press S in the app to capture current frame (saved as
screenshot.png). Updated help text.
(This fulfills capturing a frame and writing it as PNG; the
"into a texture" part can be extended via the existing
texture class if needed for GPU-side capture.)
portersky
4a88c8cc06