img, loader: add image resizing, misc fixes

* resize images with stb_image_resize
* use tee for output handle redirection (redirectToFile blocks)
* cache original image files
* accept lseek in sandbox
* misc stbi fixes

For now, I just pulled in stb_image_resize v1. v2 is an extra 150K in
size, not sure if it's worth the cost. (Either way, we can always switch
later if needed, since the API is almost the same.)

Next step: move sixel/kitty encoders to CGI, and cache their output in
memory instead of the intermediate RGBA representation.

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