[Example] SAM: support box prompt

[kwsp]

Segment Anything supports points and boxes as prompts. Currently the example only supports a single point. This PR adds

• Support for a single box
• Support for single mask output (defaults to multi mask output)

Further work (outside the scope of this PR):

• Support multiple points
• Support multiple boxes
• Support boxes and points

[ggerganov]

Thanks for the addition. Consider adding a test to ci/run.sh to exercise the new functionality.

[kwsp]

@ggerganov Added a test for the 2 new features to ci/run.sh!

[kwsp]

I added these features while porting MedSAM to ggml. MedSAM uses different preprocessing and only supports box prompt and single mask output.

Here's that repo: https://github.com/kwsp/MedSAM.cpp, linked here for reference. Since my codebases use OpenCV, I replaced the manual bilinear interpolation and sam_image_* with OpenCV, so you probably don't want that here 😜