[FEATURE REQUEST]Read as little as possible from image during 'open' call

When scanning a drive for images, I am interested in gathering all images that are larger than a specific size. During the open call, more info than just the header is read in, slowing the call significantly. Calling open on thousands or millions of images is very time-consuming.

For example, in
```
auto in = ImageInput::open (filename);
if (! in)
return;
const ImageSpec &spec = in->spec();
int xres = spec.width;
int yres = spec.height;
```
the TGA processing reads in the thumbnail data.

I would like to have all pixel processing, even the thumbnails, removed from the open call and just have the minimal spec information read so that this call is performant.

Start at the ImageInput::open entry point and follow the TGA processing path, using the ImageSpec access shown in the report. Measure which data the open call reads for large batches of images. Done means opening an image reads only the minimal spec information, without pixel or thumbnail data, while the existing API behavior remains usable.

Written by the indexing model from the issue text.