DMI metadata reading and injection

[Y0SH1M4S73R]

This PR adds rustg_dmi_read_metadata to read metadata from a DMI, and rustg_dmi_inject_metadata to inject DMI metadata into a PNG-formatted file.

With rustg_dmi_inject_metadata, in conjunction with dmi_create_png's new support for alpha channels, it will now be possible to create arbitrary dmis from user-provided pixel data much faster than with BYOND's own icon operations.

Example Usage:

var/list/pixels = /* Some list containing 4096 rgba strings - 128 rows of 32 pixels, 32 rows per dir. */
rustg_dmi_create_png("output.dmi", "32", "128", pixels)
var/metadata = json_encode(list(\
  "width" = 32,\
  "height" = 32,\
  "states" = list(\
    list(\
      "name" = "",\
      "dirs" = 4,\
    )\
  )\
)
rustg_dmi_inject_metadata("output.dmi", metadata)

[ZeWaka]

seems a bit strange to add a way to inject without an ability to read

[Y0SH1M4S73R]

seems a bit strange to add a way to inject without an ability to read

Well if you want a DMI metadata reading function to go with it, I can do that.