Fix an edge case with loadImage in Safari and older Firefox

[mourner]

Fixes #10180 by getting data out of an Image object eagerly when running loadImage in case ImageBitmap is not supported.

In the stretchable images example, loadImage result was saved to be used later (when both images are loaded), and there could be a situation where the image memory is freed (with the transparent image hack) before it gets passed to addImage. This PR fixes this by doing getImageData as soon as we load the image, not later at the addImage stage, when using the combination of loadImage and addImage with a delay in between.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality — need to see if it's possible to unit-test this properly
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fixed a bug where map.loadImage followed by map.addImage with a delay could fail in Safari and Firefox.</changelog>

[ghost]

Will this be backported to 1.13?

[vepr-ua]

This would be a good idea to be backported to 1.13. We can't switch to v2 as we need IE support and the v1 worked just fine.

[mourner]

@VeprUA this was already backported and included in v1.13.1, unless you're hitting another related issue?

[vepr-ua]

Appreciate this. Sorry didn't see there was a new version update.