refactor: share GUI helpers between image and video examples

[PABannier]

What

examples/main_image.cpp and examples/main_video.cpp each had their own identical copy of:

• INSTANCE_COLORS + N_COLORS
• basename_of
• apply_theme (53 lines)
• upload_texture

These now live in one header-only examples/common.h (still static, unchanged), and both files include it.

Why

A theme or texture-upload change was having to be made twice, and the two copies could drift apart. Removes 158 duplicated lines.

Details

• common.h includes <SDL.h>, <SDL_opengl.h>, <imgui.h>, <cstdint> and <string>, in the same order as the .cpp files. It relies on the same SDL/imgui targets (imgui-sdl2, SDL2::SDL2), so nothing changes for the Windows/vcpkg or Linux CI builds.
• #include "common.h" goes after each file's system includes, so SDL_MAIN_HANDLED is still defined before <SDL.h> is first included.
• No CMake change: the header sits next to the sources.
• Nothing else was touched: screen_to_image, create_tracker, the app-state structs and all other lines are the same as before.

Verification

• Full Release build on macOS (SDL2 installed): sam3_image and sam3_video both build. No errors and no new warnings. The only warnings are existing ones from ggml and stb.
• Byte-identity: diff of the header body against the original blocks in both files shows no difference.
• git diff of the .cpp files: the only added line in each is #include "common.h".
• Every moved helper is used in both files, so there are no unused-static warnings.

$ git diff --shortstat origin/main
 3 files changed, 90 insertions(+), 158 deletions(-)

Caveats

• Only built locally on macOS. The Windows (MSVC + vcpkg SDL2) and Linux builds are covered by CI.
• Other cleanup PRs edit lines of main_video.cpp near the removed upload_texture block (around screen_to_image / create_tracker). If they merge first, expect small context-only rebase conflicts.

🤖 Generated with Claude Code

https://claude.ai/code/session_01M2qLmQ9ag49hT61V6i9qAq