Implement transparency in MGLMapSnapshotter

[fabian-guerra]

As is asserted in this comment a MGLMapView is not necessarily opaque. MGLMapSnapshotter should implement an alpha channel if the background style layer has a non-opaque fill color.

[1ec5]

This should just be a matter of passing YES in as the second parameter to UIGraphicsBeginImageContextWithOptions():

mapbox-gl-native/platform/darwin/src/MGLMapSnapshotter.mm

Line 139 in 736e9d6

UIGraphicsBeginImageContextWithOptions(mglImage.size, NO, self.options.scale);

If the image ends up getting saved out in a format that doesn't support transparency, like JPEG, the output will be no worse than it would be without this change. For images that require no alpha channel, which is the usual case, I suppose there would be a marginal increase in file size, but it shouldn't matter greatly.

[1ec5]

if the background style layer has a non-opaque fill color

This would require #9914, although per #10118 (comment), I don’t think it’s strictly necessary to enable opacity in any case.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.