Decode images out of process. This is for improved security because buffer overflows when decoding images are a real risk. This commit adds a new executable, image_decoder, which reads a compressed image on fd 0 and writes JSON describing its frames, sizes, and delays (for animated GIFs) on fd 1. When an image is received, the image_decoder job is run in a sandbox. Likewise, when iTerm2 restores saved state, image_decoder again decodes each image. In this way images that were not in our control are always passed through image_decoder. Some images, like the broken pipe graphic, are under our control and do not need to be decoded out of process. When restoring state, one image at a time is decoded in order of most recently requested. This provides adequate performance while minimizing latency.

Decode images out of process. This is for improved security because buffer overflows when decoding images are a real risk. This commit adds a new executable, image_decoder, which reads a compressed image on fd 0 and writes JSON describing its frames, sizes, and delays (for animated GIFs) on fd 1. When an image is received, the image_decoder job is run in a sandbox. Likewise, when iTerm2 restores saved state, image_decoder again decodes each image. In this way images that were not in our control are always passed through image_decoder. Some images, like the broken pipe graphic, are under our control and do not need to be decoded out of process. When restoring state, one image at a time is decoded in order of most recently requested. This provides adequate performance while minimizing latency.

Add a context menu item when you right click on an animated image that toggles its animation. Issue 4790.

Add support for insets inline images. Add optional paramters insetLeft, insetRight, insetTop, and insetBottom, which default to zero. The image will be inset within its original space. The insets are clamped to the size of a character cell. Issue 4419.

Add support for insets inline images. Add optional paramters insetLeft, insetRight, insetTop, and insetBottom, which default to zero. The image will be inset within its original space. The insets are clamped to the size of a character cell.

Add support for animated GIFs and in the process clean up some of the stuff around how images are handled:
* Move responsiblity for creating a pasteboard item and saving a temp file into ImageInfo.
* Make mouse become a cursor when over an image.
* Make cmd-click on an image open it.
* Move ImageInfo into its own file.
* Also clean up a bogus method in a PSM header.

Cleanup