Fix ImageSource for Firefox and Safari

[mactrem]

Closes #51.
This PR fixes that ImageSources are not shown on Firefox and Safari.
Fix taken from mapbox/mapbox-gl-js#10230
Fix is too simple to be copyrightable.

Launch Checklist

• manually test the debug page

[kylebarron]

Fix is too simple to be copyrightable

Do you have any legal sources to back up this claim?

[github-actions]

Bundle size report:

Size Change: +9 B
Total Size Before: 201 kB
Total Size After: 201 kB

Output file	Before	After	Change
mapbox-gl.js	196 kB	196 kB	+9 B
mapbox-gl.css	4.62 kB	4.62 kB	0 B

ℹ️ View Details

Source file	Before	After	Change
src/util/ajax.js	2.64 kB	2.66 kB	+16 B

[mourner]

No worries about the copyright — we're backporting the fix to v1.13 anyway.

[marcelnormann]

I prefer to wait for the official backport. Seems to be too small for copyright infringement, but a lot of such changes can make it one.

[lseelenbinder]

As discussed in a recent steering committee meeting, this repository's policy is to use the upstream commit to avoid any legal issues.

I will close this until we can use that commit after they release v1.13.x with it included. Appreciate your effort @mactrem; I wish we could just merge this. 😄

[kylebarron]

I will close this until we can use that commit after they release v1.13.x with it included

FYI looks like they've released a patch release https://github.com/mapbox/mapbox-gl-js/releases/tag/v1.13.1

[mourner]

@lseelenbinder yep, feel free to reopen — this was cherry-picked into the open source branch in mapbox/mapbox-gl-js#10394

[lseelenbinder]

Pulled in #87 to preserve attribution.