Memory cache Build improvement: two file reads with one input file used in two separate places with different output widths

[zachleat]

e.g. on 11ty-website:

{% image "./src/img/possum-geri.png", "The possum is Eleventy’s mascot", [350], "elv-possum" %}
{% image "./src/img/possum-geri.png", "3 possums suspended from a red balloon", [null] %}

This will add two separate entries to the memory cache (which is correct, note that widths is [350] on one and [null] on another) but performance could be improved—it reads the file contents from the file system twice!